During Ayame Takamine’s climax, a large amount of nectar surged out, drenching the tip of Ethan’s behemoth with a tingling sensation. The dense, soft, and tender flesh tightly wrapped around the shaft. But Ethan endured this soul-melting constriction, continuing to thrust in the beautiful girl’s warm, tight, rosy cave.
The pink-haired, big-breasted beauty panted weakly, her delicate eyebrows slightly furrowed, an expression of pain on her face.
Ethan stopped his movements, a look of tender affection on his handsome face.
“Sis’ Ayame, what’s wrong?”
Ayame Takamine let out a soft “Mmm,” and said shyly, “It hurts…”
Ethan kissed her, then smiled naughtily. “Really? I don’t recall you being so fragile.”
Ayame Takamine’s face flushed slightly. She was gasping and whimpering.
“No… no more… Mom… Mom is watching… I… I don’t want to become like Sayuri…”
Ethan thrust several times vigorously. The enormous tip breached her flower-palace and ground around in the sacred, tender space for a moment, causing the pink-haired beauty to have another small climax. Only then did he reluctantly withdraw.
He rolled over, pulled out his large flesh rod, reached out to pull the beautiful girl into his embrace, and kissed her deeply on the lips.
“Alright, since this is Sis’ Kasumi’s fault, then next… Sis’ Kasumi, the trouble you’ve caused is yours to solve.”
Ayame Takamine felt a moment of regret for not receiving the hot essence, but seeing her mother so close, the shame outweighed her lust, which had cooled considerably after her climax.
She hugged Kasumi Takamine from behind, her slender hands caressing the pair of snow-white giant breasts that had once nourished her and Sayuri Takamine.
The beautiful, mature widow had been hot and restless watching Ethan and her daughter’s passionate dual cultivation.
Compared to Ayame Takamine, who appeared open but was actually thin-skinned, she was just as obsessed with this matter as Sayuri.
No, it should be said that my elder daughter, Ayame Takamine, inherited my external personality, while my younger daughter, Sayuri Takamine, inherited my true inner nature.
Using the pink-haired, big-breasted beauty’s action as an excuse, Kasumi Takamine let out a soft “Mmm,” her snow-white, voluptuous body collapsing entirely into Ethan’s arms.
“Little Ethan… kiss me…”
Ethan kissed her lightly. “Sis’ Kasumi, be prepared.”
Kasumi Takamine was about to respond when her seductive eyes suddenly widened. “Oh! Little Ethan… you’re so naughty! Slow down… Mmm…”
The hot behemoth, with Ayame Takamine’s help, had already pressed against the entrance of her wet, tender honey-hole.
Being sandwiched between her daughter and adopted son, Kasumi Takamine’s breathing became rapid and shallow.
Ethan constantly kissed her earlobe, whispering softly in her ear: “Sis’ Kasumi, Auntie Kasumi… Mother-in-Law… Mom Kasumi…”
As he spoke increasingly provocative titles, he moved the behemoth back and forth, repeatedly rubbing against the entrance of the hot, slick flower-valley.
The mature, beautiful widow anxiously twisted her slender waist, trying to swallow the hot behemoth and soothe the ache and heat in her body.
“Little Ethan… hurry… hurry and come in… I want you…”
Ethan buried his head, kissing Kasumi Takamine’s beautiful breast peaks, lightly sucking the luscious bud at the tip.
The tender bud was already erect, like a ripe little grape, tasting slightly sweet in his mouth. Ethan constantly licked and sucked the cute bud.
The tip of his penis felt Kasumi Takamine’s sensitive flower-valley becoming increasingly moist and slick. He tentatively thrust forward. The tight passage was easily parted, then immediately clamped down again, as if welcoming a distinguished guest.
Kasumi Takamine lay weakly in Ethan’s embrace, her smooth, beautiful legs entwining his. She let out a low sigh and moan.
Ethan hugged her slender waist and thrust hard. More than half of the behemoth squeezed in.
Kasumi Takamine moaned, wrapping her arms around Ethan’s waist. “It reached the top, so deep… don’t go in any further…”
The sensitive tip felt the biting suction of her flower-palace entrance. Ethan’s heart stirred, unable to suppress the desire to fully conquer the sacred flower-palace of the pure widow.
He thrust his hips slightly. The enormous tip struck the entrance of the flower-palace, eliciting muffled groans from Kasumi Takamine.
“Sis’ Kasumi… Sis’ Ayame and Sis’ Sayuri can both take it all. See…”
Ayame Takamine panted softly, subconsciously looking at their joining point. Sure enough, three to four centimeters of the shaft was exposed to the air.
“Mmm… how can it be so big… I… I can’t… if you go in any further, I’ll die…”
Her face was crimson, her final words barely a whisper.
“But… Ayame and Sayuri can do it… I… I want to try too…”
Ethan thrust his hips. The behemoth’s tip broke through the layers of tender vaginal flesh and plunged straight onto the flower-heart. The warm, tight, enveloping sensation made him shiver comfortably.
“Sis’ Kasumi, don’t worry, I have experience. The first time might be a bit uncomfortable, but after that, it will feel very good.”
Yesterday, fearing he might hurt the widow who hadn’t been intimate for over a decade, Ethan had held back. Now, he could finally complete the last step he hadn’t taken yesterday.
Compared to the girl’s delicate body, the mature, voluptuous body was indeed easier to open up. Ethan plunged deep into Kasumi Takamine’s flower-valley with force, the tip striking her flower-heart squarely with every thrust.
Gradually, the behemoth sank deeper and deeper, and the exposed part grew smaller.
Kasumi Takamine’s eyes were seductive like spring water, her nose covered in fine sweat. Transparent, sticky nectar flowed down her snow-white thighs.
“Be gentler… can you be gentler… Ah… Little Ethan… too forceful, ah…”
Her mature body was stimulated by these deep thrusts, causing her to cry out with shy, alluring tones. She involuntarily spread her legs wider, her hands tightly hugging Ethan’s waist.
Ethan plunged deep into her tight honey-hole, supporting her round, perky buttocks with his hands, making her pale flower-valley more prominent. Her thigh muscles tensed rigidly, supporting the ferocious thrusting of little Ethan.
The insertion of the behemoth slightly depressed her honey-hole. The rapid withdrawal brought out droplets of nectar from their point of union, splashing onto the bedsheet.
“Ah, ah… so… so comfortable?… Ah…”
Kasumi Takamine’s uninhibited moans, the fleshy sound of skin colliding, and the squelching sound of their lower bodies grinding formed an enticing symphony.
She tightly embraced Ethan. Her smooth legs, coordinating with her body’s undulations, rubbed against Ethan’s body. Her soft hands pressed down on Ethan’s taut buttocks.
Her eyes were half-closed, her gaze swirling with seductive allure. Her long, thick eyelashes constantly fluttered, and her eyebrows were slightly furrowed, yet her face showed the joyous passion of spring. Her rosy lips appeared vivid and tender.
The behemoth, heavily coated in nectar, fiercely assaulted Kasumi Takamine’s snow-white, voluptuous body. The beautiful widow could no longer utter a complete sentence.
“Ah… ah, good… slow down, ah…”
She moaned haphazardly, her fingers tightly grasping Ethan’s back. The full, mature breasts on her beautiful body pressed against Ethan’s chest with the thrusting motion.
Ayame Takamine was not idle. To lessen her mother’s suffering, she constantly moved her slender fingers and tender tongue over Kasumi Takamine’s sensitive points on her back.
The pink-haired, big-breasted beauty occasionally used her delicate, fragrant tongue to lick the beautiful wife’s snowy neck, sometimes blew air into her flushed ears, and occasionally sucked and licked her crystalline, jade-like earlobe.
She also used her slender, jade hands to roam over Kasumi Takamine’s delicate body, her right index finger constantly touching and probing the tender anal flower.
With virtually all sensitive points simultaneously assaulted, Kasumi Takamine, as a mother, had never experienced such intensity. Her mature body completely went limp, letting her adopted son have his way with her.
Ethan used both hands to lift her smooth, muscular, perky buttocks, raising her flower-valley high, forcefully impacting the slightly opened flower-palace entrance.
Little Ethan felt the narrow passage contract with each thrust. The inner gate opened, sucking the tip. Seizing the opportunity, Ethan thrust with all his might. The smooth, enormous tip broke through the barrier, completely entering the sacred flower-palace.
“Ah…”
Kasumi Takamine first let out a cry of surprise. A look of pain crossed her flushed face. As the behemoth fully entered, the thirty-something, seductive widow seemed to undergo a second deflowering.
But soon, with the in and out movement of the behemoth, she felt an unprecedented pleasure. Her small hands felt the contours of the behemoth through a layer of membrane. The sheer joy of being completely filled assaulted Kasumi Takamine’s mind.
Her tightly furrowed brow slowly relaxed. Unprecedented pleasure took the highest ground. Her beautiful body began to tremble. One uncontrollable shiver after another accompanied intense pleasure releasing through her body.
“Don’t stop… don’t stop… Ah, it’s coming?… Ah…”
Kasumi Takamine arched her snow-white neck, her curly chestnut hair cascading like water. Her fingers fiercely dug into Ethan’s muscle.
The force, strong enough to draw blood and bruise an ordinary person, could only barely leave red marks on Ethan’s firm body.
Ethan thrust excitedly and powerfully. The hot, thick behemoth deeply penetrated the sacred flower-palace. The smooth tip constantly rotated and ground against the tender palace walls, sending the trembling mature body to a higher wave crest.
Source: Webnovel.com, updated by NovelKeep
Chapters
- Chapter 344: The Correct Way to Use Telekinesis
- Chapter 343: Sparring
- Chapter 342: Full Party Level-Up
- Chapter 341: Taking Turns Cultivating
- Chapter 340: The Talents of the Naruse Sisters
- Chapter 339: Subduing the ’Bratty’ Succubus
- Chapter 338: Shota Ethan
- Chapter 337: Maria, Transform!
- Chapter 336: One After Another
- Chapter 335: The Other Side of Kurumi
- Chapter 334: Kurumi
- Chapter 333: Mio
- Chapter 332: Mio’s Turn
- Chapter 331: Yuki’s Turn
- Chapter 330: Sheera and Lucia
- Chapter 329: Looting the Estate
- Chapter 328: Psychokinetic Magic
- Chapter 327: The Rescue
- Chapter 326: Zest
- Chapter 325: Lars
- Chapter 324: Simultaneous Progression
- Chapter 323: Two Methods
- Chapter 322: Maria’s Conflict
- Chapter 321: Change of Surveillance
- Chapter 320: Another Mission
- Chapter 319: Each Getting What They Need
- Chapter 318: The Past & The Goal
- Chapter 317: The Nonaka Sisters
- Chapter 316: Ultra Devil King Class
- Chapter 315: Yuuki & Asia
- Chapter 314: Yuuki & Asia
- Chapter 313: Yuuki & Asia
- Chapter 312: Yuuki & Asia (Part 2)
- Chapter 311: Yuuki & Asia (Part 1)
- Chapter 310: The Holy Sword Durandal
- Chapter 309: The Blonde, Busty Big Sister
- Chapter 308: Battle Form
- Chapter 307: The Enemy Appears
- Chapter 306: Valper Galilei
- Chapter 305: Cooperation
- Chapter 304: You Have to Trust Your Mother
- Chapter 303: Mrs. Hyoudou
- Chapter 302: An Encounter with a "Cuckold" Scene
- Chapter 301: The Holy Sword Project and "Yuuki"
- Chapter 300: Hunting and the Holy Swords
- Chapter 299: Demon King Class
- Chapter 298: Satisfaction
- Chapter 297: The Return of Katerea
- Chapter 296: Return & Expansion
- Chapter 295: Shared Intensity
- Chapter 294: Karamine
- Chapter 293: Siris
- Chapter 292: Plans for the Girls & The Second Night
- Chapter 291: All in the Name of Healing
- Chapter 290: Minami’s Turn
- Chapter 289: The Pure and Innocent Minami
- Chapter 288: Yubelluna’s Turn
- Chapter 287: House Calls and Healing
- Chapter 286: Maria’s Potential
- Chapter 285: The Peerage Slots
- Chapter 284: Ethan’s Evil Pieces
- Chapter 283: Departure & Resistance
- Chapter 282: Successful Correction?!
- Chapter 281: The Thirsty Lady Leviathan
- Chapter 280: Correction in Progress
- Chapter 279: The Dark-Skinned ’Bitch’
- Chapter 278: The ’Ten Scent Tendon-Weakening’ Drug
- Chapter 277: The Peace Treaty & The Captive’s Fate
- Chapter 276: Capturing Katereia
- Chapter 275: The Battle’s End
- Chapter 274: Ethan’s Balance Breaker
- Chapter 273: The Self-Cultivation of an Actor
- Chapter 272: Facing the White Dragon Emperor
- Chapter 271: The Battle Begins & Zorga Appears
- Chapter 270: Ornis & Kokabiel
- Chapter 269: The Emergence of Khaos Brigade
- Chapter 268: The Peace Treaty and Its Problems
- Chapter 267: Above God-Class
- Chapter 266: Prelude to the Summit
- Chapter 265: Ni & Li (Part 3)
- Chapter 264: Ni & Li (Part 2)
- Chapter 263: Ni & Li (Part 1)
- Chapter 262: Mira
- Chapter 261: Shuriya’s Turn
- Chapter 260: The White Tiger and the Green Dragon
- Chapter 259: The Female Constraint and a New Round of Healing
- Chapter 258: The ‘Polluted’ Pieces
- Chapter 257: Epiphany and SSS-Rank
- Chapter 256: Gasper Vladi
- Chapter 255: Case Closed and Idle Rumors
- Chapter 254: The Breast-Sucking… Blood-Sucking Plant!
- Chapter 253: Physical Inspection
- Chapter 252: Gasper
- Chapter 251: The Search
- Chapter 250: A Vampire?
- Chapter 249: Burent
- Chapter 248: Continuing the Treatment
- Chapter 247: Marion
- Chapter 246: Don’t Waste Time
- Chapter 245: Yiru’s Turn
- Chapter 244: Neru’s Turn
- Chapter 243: Healing Assistance
- Chapter 242: Transfer Student +1
- Chapter 241: Fierce Battle
- Chapter 240: Benefits for the Future Brother-in-Law
- Chapter 239: The Challenge Letter
- Chapter 238: Moving?
- Chapter 237: The Tojo Family Business
- Chapter 236: The Dust Settles
- Chapter 235: Refusing to Surrender
- Chapter 234: The Opponents are Out (Part 2)
- Chapter 233: The Opponents are Out (Part 1)
- Chapter 232: The Crushing Victory
- Chapter 231: Opponents Divided
- Chapter 230: Game Start
- Chapter 229: Pre-Game Preparation
- Chapter 228: Sona Shitori’s Kindness
- Chapter 227: Contract Complete
- Chapter 226: Contracting Yuma Amano
- Chapter 225: The Silver Crest
- Chapter 224: Contracting Akeno Himejima
- Chapter 223: Contract Slots
- Chapter 222: Contract Variation
- Chapter 221: Contracting Rias
- Chapter 220: The Succubus Contract
- Chapter 219: Settlement and Rias’s Return
- Chapter 218: The Rescue, Part 2
- Chapter 217: The Rescue, Part 1
- Chapter 216: The Threat
- Chapter 215: You Are a Devil, Too~
- Chapter 214: The Wary Succubus LOLI
- Chapter 213: Search for the Contract and the Naruse Sisters
- Chapter 212: Challenge and Contract
- Chapter 211: End of Special Training and the Phenex
- Chapter 210: The Exclusive Seat
- Chapter 209: Transformation and Berserk
- Chapter 208: Koneko’s Resolution
- Chapter 207: The Assertive Loli
- Chapter 206: The Charging Loli
- Chapter 205: Akeno Himejima’s Special Training, Part 3
- Chapter 204: Akeno Himejima’s Special Training, Part 2
- Chapter 203: Akeno Himejima’s Special Training, Part 1
- Chapter 202: The Third Phase of Special Training
- Chapter 201: Undercurrents
- Chapter 200: Special Training
- Chapter 199: Enthusiastic Classmates
- Chapter 198: Lovers?
- Chapter 197: Playing in the Water
- Chapter 196: Rias’s Breakthrough
- Chapter 195: Awakening
- Chapter 194: Rias’s Turn
- Chapter 193: The Candidate
- Chapter 192: Three Solutions
- Chapter 191: Grayfia’s Examination (Part II)
- Chapter 190: Grayfia’s Examination (Part I)
- Chapter 189: Juggernaut Drive & Extinguish Star
- Chapter 188: Neither Hard Nor Soft
- Chapter 187: The Collaboration Begins
- Chapter 186: Bishop Asia
- Chapter 185: Grayfia’s Debut
- Chapter 184: My Fault, Then?
- Chapter 183: Yuuto Kiba’s Secret
- Chapter 182: The New Servant Candidate
- Chapter 181: Abducting the Little Nun
- Chapter 180: Asia Argento
- Chapter 179: Investigation in Progress
- Chapter 178: Injury
- Chapter 177: Azazel
- Chapter 176: Departure
- Chapter 175: New Talent and Spring Break
- Chapter 174: Another Organization
- Chapter 173: The Emergence of an Idea
- Chapter 172: The Relay Race (Part 2)
- Chapter 171: The Relay Race (Part 1)
- Chapter 170: Conquest
- Chapter 169: Tsukino Yomizaki
- Chapter 168: The “Coincidence”
- Chapter 167: Mizuki Suzumori (Part 2)
- Chapter 166: Mizuki Suzumori (Part 1)
- Chapter 165: A Tsundere Rias
- Chapter 164: Two Options
- Chapter 163: Akane Nanao (Part 2)
- Chapter 162: Akane Nanao (Part 1)
- Chapter 161: Continuing the Objective
- Chapter 160: Twin Blooms
- Chapter 159: The Sisters
- Chapter 158: Practical Lesson
- Chapter 157: Where to Go
- Chapter 156: The Reason for the Signature
- Chapter 155: The Sakuramiya Sisters
- Chapter 154: The Next One
- Chapter 153: Conquest
- Chapter 152: Continuation
- Chapter 151: Identical Talents
- Chapter 150: Natsuha Murasakimiya, the Lustful Maiden
- Chapter 149: A Little Competition?
- Chapter 148: Valentine’s Day
- Chapter 147: Reopening Old Goals
- Chapter 146: Team Battle
- Chapter 145: Nue, Kuma, and Baku
- Chapter 144: Miura Corp. and the Sanda-kai
- Chapter 143: Investigation
- Chapter 142: Blocked at the Door
- Chapter 141: The Trouble with Resignation
- Chapter 140: The Whereabouts of My Parents
- Chapter 139: Winter Break Countdown
- Chapter 138: The Bowl (Finale)
- Chapter 137: The Bowl (Part 3)
- Chapter 136: The Bowl (Part 2)
- Chapter 135: The Bowl (Part 1)
- Chapter 134: The Prelude
- Chapter 133: Magic Affinity
- Chapter 132: Kasumi Takamine’s Scene
- Chapter 131: The War Beneath the Table
- Chapter 130: The Return of Kasumi Takamine
- Chapter 129: Longinus Get!
- Chapter 128: The Plan, Part II
- Chapter 127: The Plan, Part I
- Chapter 126: Ultra-Thin Black Sheer
- Chapter 125: Escape and S-Rank
- Chapter 124: The Sky as a Blanket, the Earth as a Bed
- Chapter 123: Taking Turns
- Chapter 122: The Foot Fetish Pervert…
- Chapter 121: Assisting the Breakthrough
- Chapter 120: Attacking the Barrier
- Chapter 119: Large Barrier
- Chapter 118: Mitelt’s Disappearance
- Chapter 117: The Deprivation Ritual
- Chapter 116: A-Rank Peak
- Chapter 115: Talent: [Instant Illusion]
- Chapter 114: Second Bloom
- Chapter 113: Amano Yuuma’s Turn – Part III
- Chapter 112: Amano Yuuma’s Turn – Part II
- Chapter 111: Amano Yuuma’s Turn – Part I
- Chapter 110: Mitelt’s Turn
- Chapter 109: Kalawarner’s Turn
- Chapter 108: Kalawarner, You Go First, I’m Scared…
- Chapter 107: Mitelt and Kalawarner
- Chapter 106: Six and Nine
- Chapter 105: The First Taste
- Chapter 104: Back Scrubbing with Rias
- Chapter 103: The Pure Fallen Angel
- Chapter 102: Caleb’s Devil Form
- Chapter 101: Subjugation Mission
- Chapter 100: The Catalyst for Breakthrough
- Chapter 99: Ranking Battle
- Chapter 98: Perfect Score
- Chapter 97: Test Commencement
- Chapter 96: New Semester
- Chapter 95: Sub-Dragon God Tier
- Chapter 94: The Student Roster
- Chapter 93: Akino Sakagami’s Case
- Chapter 92: Rena Kurashiki’s Session
- Chapter 91: Yui Obata
- Chapter 90: Rias’s First Kiss
- Chapter 89: Koneko’s Thoughts
- Chapter 88: Club Activities Concluded and the Crest
- Chapter 87: Club Activity Part 3
- Chapter 86: Club Activity Part 2
- Chapter 85: Club Activity Part 1
- Chapter 84: Sacred Gear
- Chapter 83: The Replacement
- Chapter 82: Yūma Amano’s Curiosity
- Chapter 81: Yūma Amano
- Chapter 80: Mirror Reflection
- Chapter 79: Tsubaki Shinra’s Master
- Chapter 78: Cultivation Results
- Chapter 77: The Cultivation in Progress…
- Chapter 76: Sakura Miyajima’s Cultivation
- Chapter 75: Target Located
- Chapter 74: Method of Cultivation
- Chapter 73: Mayuna’s Night
- Chapter 72: Miyajima Family Council (Part 2)
- Chapter 71: Miyajima Family Council (Part 1)
- Chapter 70: Thrice to the Gate of… School
- Chapter 69: Officially Joining
- Chapter 68: I Won’t Trouble You, Akeno-senpai!
- Chapter 67: The Incident
- Chapter 66: Sakura Miyajima
- Chapter 65: Aiho Academy
- Chapter 64: Tsubaki Miyajima’s Unmet Desires
- Chapter 63: Tsubaki Miyajima
- Chapter 62: Incident at the Kendo Dojo
- Chapter 61: The Spoils of War
- Chapter 60: The Succubus Reveals Her Ecstatic State
- Chapter 59: I Am a Succubus!
- Chapter 58: Impatient Touko Sakurauchi
- Chapter 57: Forbidden Romance
- Chapter 56: The Ten Principal Gods
- Chapter 55: Chisato Hasegawa
- Chapter 54: Unexpected Turns
- Chapter 53: Encountering Issei Hyodo Again
- Chapter 52: Exploring Other Methods
- Chapter 51: Dual Cultivation Ability – Part 3
- Chapter 50: Dual Cultivation Ability – Part 2
- Chapter 49: Dual Cultivation Ability – Part 1
- Chapter 48: Enchantment
- Chapter 47: I Thought You Liked It…
- Chapter 46: Following Suit
- Chapter 45: Complete Bloodline Talent
- Chapter 44: Vice President – Tsubaki Shinra
- Chapter 43: Defiled
- Chapter 42: The Contract
- Chapter 41: Off Target
- Chapter 40: The Decision (Part 2)
- Chapter 39: The Decision (Part 1)
- Chapter 38: Magic Genius?!
- Chapter 37: Petite
- Chapter 36: Such White Thighs
- Chapter 35: Little Caleb
- Chapter 34: Koneko Toujou
- Chapter 33: Rank Classification
- Chapter 32: Tracing the Source
- Chapter 31: The Reason
- Chapter 30: Surrender and Lose Half
- Chapter 29: True Man 1v1 Battle
- Chapter 28: The Woods
- Chapter 27: The First Target
- Chapter 26: Bloodline Talent
- Chapter 25: Afternoon Tea
- Chapter 24: Talent: Mirror Reflection (Partial)
- Chapter 23: The Investigation
- Chapter 22: Familiar
- Chapter 21: Evil Pieces and Rating Games
- Chapter 20: She Falls for This Trick
- Chapter 19: The Crimson-Haired Ruin Princess
- Chapter 18: Aptitude
- Chapter 17: The Stalker
- Chapter 16: Assessment Concluded
- Chapter 15: Familiar Faces
- Chapter 14: The Targets
- Chapter 13: Mid-Month Assessment
- Chapter 12: Lilith Takamine
- Chapter 11: Claimed by Others
- Chapter 10: The Mixed-Bloods
- Chapter 9: The Takamine Household
- Chapter 8: The So-Called Punishment
- Chapter 7: The Golden-Haired Beauty
- Chapter 6: You Wouldn’t Want Them to Know, Would You?
- Chapter 5: The Gender-Bending Bully
- Chapter 4: The Succubus Bloodline
- Chapter 3: The Dual Cultivation System
- Chapter 2: The Awakening of Power
- Chapter 1: A World in Chaos