- O.M.G. 26: Our Mini Games (DS)
- Observer (Switch eShop)
- Ocean Runner (3DS eShop)
- Oceanhorn: Monster of Uncharted Seas (Switch eShop)
- Ochaken no Daibouken 2: Yume Ippai no Omocha Hako (DS)
- Ochaken no Daibouken: Honwaku Yumemiru Sekai Ryoukou (DS)
- Ochaken no Heya DS (DS)
- Ochaken no Heya DS 2 (DS)
- Ochaken no Heya DS 3 (DS)
- Ochaken no Heya DS 4 (DS)
- Octahedron: Transfixed Edition (Switch eShop)
- Octocopter: Double Or Squids (Switch eShop)
- Octocopter: Super Sub Squid Escape (Wii U eShop)
- Octodad: Dadliest Catch (Switch eShop)
- Octodad: Dadliest Catch (Wii U eShop)
- Octogeddon (Switch eShop)
- Octomania (Wii)
- Octopath Traveler (Switch)
- Odallus: The Dark Call (Switch eShop)
- Odama (GCN)
- Oddworld Adventures (GB)
- Oddworld: New 'n' Tasty (Wii U eShop)
- Oddworld: Stranger's Wrath (Switch)
- Oddworld: Stranger's Wrath HD (Wii U eShop)
- Odium to the Core (Switch eShop)
- Oekaki Puzzle Battle Vol. 1: Yuusha-Oh GaoGaiGar Version (DS)
- Of Mice and Sand (3DS eShop)
- Of Mice and Sand -Revised- (Switch eShop)
- Ogre Battle 64: Person of Lordly Caliber (N64)
- Ogre Battle: The March of the Black Queen (SNES)
- Oh My Godheads: Party Edition (Switch eShop)
- Oh!Edo Towns (Switch eShop)
- Oh...Sir! The Hollywood Roast (Switch eShop)
- Oh...Sir! The Insult Simulator (Switch eShop)
- Oha Star 645: Musshees no Fushigi na Nouen - Yoshimoto Geinin to Tomodachi Net (DS)
- Ohayou! Beginner's Japanese (Switch eShop)
- Ohayou! Beginner's Japanese (Wii U eShop)
- Ohno Odyssey (3DS eShop)
- OK K.O.! Let's Play Heroes (Switch eShop)
- OK K.O.! Let’s Play Heroes (Switch eShop)
- Okaeri! Chibi-Robo! Happy Richie Oosouji! (DS)
- Okami (Wii)
- Okami HD (Switch eShop)
- Okamiden (DS)
- OkunoKA (Switch eShop)
- Old Man's Journey (Switch eShop)
- Old School Musical (Switch eShop)
- Old School Racer 2 (Switch eShop)
- Old Time Hockey (Switch eShop)
- Oliver and Spike: Dimension Jumpers (Wii U eShop)
- OlliOlli (3DS eShop)
- OlliOlli (Wii U eShop)
- OlliOlli: Switch Stance (Switch eShop)
- Olympia Rising (Wii U eShop)
- Olympic Games Tokyo 2020: The Official Video Game (Switch eShop)
- Olympic Summer Games: Atlanta 1996 (GB)
- Omega Labyrinth Life (Switch eShop)
- Omega Strike (Switch eShop)
- Omensight: Definitive Edition (Switch eShop)
- OMG Zombies! (Switch eShop)
- Omvorm (Switch eShop)
- One Eyed Kutkh (Switch eShop)
- One More Dungeon (Switch eShop)
- One Piece Grand Adventure (GCN)
- One Piece Romance Dawn (3DS)
- One Piece Unlimited Cruise SP (3DS)
- One Piece Unlimited World Red (3DS)
- One Piece Unlimited World Red (Wii U)
- One Piece Unlimited World Red - Deluxe Edition (Switch)
- One Piece: Gear Spirit (DS)
- One Piece: Gigant Battle! (DS)
- One Piece: Gigant Battle! 2 - New World (DS)
- One Piece: Pirate Warriors 3 Deluxe Edition (Switch)
- One Piece: Super Grand Battle! X (3DS)
- One Strike (Switch eShop)
- Onechanbara: Bikini Zombie Slayers (Wii)
- Onegai My Melody (DS)
- Onigiri (Switch eShop)
- Oniken: Unstoppable Edition (Switch eShop)
- Onimusha Tactics (GBA)
- Onimusha: Warlords (Switch eShop)
- ONINAKI (Switch)
- Onslaught (WiiWare)
- Ontamarama (DS)
- oOo: Ascension (Switch eShop)
- Oops! Prank Party (Wii)
- Open Season (DS)
- Open Season (Wii)
- Opera Web Browser (DS)
- Opera Web Browser (Wii)
- Operation C (GB)
- Operation COBRA (3DS eShop)
- Operation Hardcore (Switch eShop)
- Operation Pig (Switch eShop)
- Operation Wolf (NES)
- Operation: Vietnam (DS)
- Opoona (Wii)
- OPUS Collection: The Day We Found Earth + Rocket of Whispers (Switch)
- OPUS: Rocket of Whispers (Switch eShop)
- OPUS: The Day We Found Earth (Switch eShop)
- ORBIT (Wii U eShop)
- Orcs & Elves (DS)
- Order Up!! (3DS eShop)
- Ordyne (TG-16)
- Original Frisbee Disc Sports: Ultimate & Golf (DS)
- Original Journey (Switch eShop)
- Orion's Odyssey (DSiWare)
- Oscar in Movieland (DSiWare)
- Oscar in Toyland (DSiWare)
- Oscar in Toyland 2 (DSiWare)
- Oscar's World Tour (DSiWare)
- Oshare Majo: Love and Berry (DS)
- Oshare Princess DS: Oshare ni Koi Shite 2 (DS)
- Oshare Princess DS: Oshare ni Koishite! (DS)
- Ostwind - Das Spiel (Switch)
- Osu! Tatakae! Ouendan (DS)
- Othello (GB)
- Othello (Switch eShop)
- Otona no Onnaryoku Kentei (DS)
- Otto (Switch eShop)
- Ottos Ottifanten: Baby Brunos's Nightmare (GB)
- OTTTD: Over The Top Tower Defence (Switch eShop)
- Our Flick Erasers (Switch eShop)
- Our House (DS)
- Our World is Ended (Switch)
- Out of Gas (GB)
- Out of The Box (Switch eShop)
- Out There: Ω The Alliance (Switch eShop)
- Outback Pet Rescue 3D (3DS eShop)
- Outdoors Unleashed: Africa 3D (3DS eShop)
- Outlast 2 (Switch eShop)
- Outlast: Bundle of Terror (Switch eShop)
- Outlaw Golf (GCN)
- Outside the Realm (Wii U eShop)
- Over the Hedge (DS)
- Over the Hedge: Hammy Goes Nuts! (DS)
- Over Top (Neo Geo)
- Overcooked 2 (Switch)
- Overcooked: Special Edition (Switch eShop)
- Overdriven Reloaded: Special Edition (Switch eShop)
- Overflow (WiiWare)
- Overland (Switch eShop)
- Overlord: Dark Legend (Wii)
- Overlord: Minions (DS)
- Overpass (Switch)
- OVERTURN (WiiWare)
- Overwhelm (Switch eShop)
- Overworld Defender Remix (Wii U eShop)
- OVIVO (Switch eShop)
- Owlboy (Switch eShop)
- Oxenfree (Switch eShop)
- Oxyjet (Switch eShop)