| Pachislot |  Okinawa Palace | 2003 | Pioneer Co., Ltd. Japan |
| Slot Machine | Oktoberfest | 2003 | IGT International Game Technologies Reno, NV, USA |
| Medal game (video) | Olando | 2003 | Unknown Korea |
| Slot Machine | Old Glory | 2003 | Cadillac Jack Duluth, GA, USA |
| Pachislot | Omatsuri-Mambo | 2003 | Aristocrat Technologies Co., Ltd. Tokyo, Japan |
| Pachinko | Omedetou Gozaima | 2003 | Newgin Japan |
| Sony PlayStation CD |  One Piece - Grand Battle Model SLES-03807 | 2003 | Bandai Co., Ltd. Japan |
| Nintendo GBA cart. | Onimusha Tactics Model AGB-A6OE-USA | 2003 | Capcom USA, Inc. Santa Clara, CA, USA 13388 |
| Nintendo GBA cart. | Onimusha Tactics Model AGB-A6OJ-JPN | 2003 | Capcom Co., Ltd. Japan |
| Nintendo GBA cart. | Onimusha Tactics Model AGB-A6OP | 2003 | Capcom Co., Ltd. Japan |
| Sony PlayStation 2 disc | Only You - Liber Cross Model SLPS-25237 | 2003 | Alicesoft Japan |
| Pachinko | Oozora Monogatari - Toripuruuingusu | 2003 | Sansei R&D Japan |
| Slot Machine | Open the Vault | 2003 | WMS Gaming, Inc. Chicago, IL, USA |
| Nintendo GBA cart. | Operation Armored Liberty Model AGB-BAAE-USA | 2003 | Majesco Sales, Inc. |
| Arcade Video game | Operation Blockade | 2003 | Global VR San Jose, CA, USA |
| Slot Machine | Orca Classics | 2003 | IGT International Game Technologies Reno, NV, USA |
| Nintendo GBA cart. | Oriental Blue - Ao no Tengai Model AGB-AORJ-JPN | 2003 | Hudson Soft. Hudson Group Japan |
| Sega Pico game | Oryouri Daisuki! Kitchen Pico | 2003 | SEGA Toys |
| Pachislot | Osaruno Chogoku | 2003 | Eleco, Ltd. Aruze Group Japan |
| Nintendo GBA cart. | Oshare Princess 3 Model AGB-BO3J-JPN | 2003 | Culture Brain |
| Microsoft XBOX disc | Otogi 2 - Immortal Warriors | 2003 | Unknown |
| Sony PlayStation CD | Otogi Story - Tenshi no Shippo Model SLPS-03531 | 2003 | Bandai Co., Ltd. Japan |
| Pachislot | Ougonshin Model G | 2003 | Olympia Co., Ltd. Japan |
| Pachislot | Ougonshin Model G Alt. Art | 2003 | Olympia Co., Ltd. Japan |
| Sony PlayStation CD | Oujisama Lv1.5 - Oujisama no Tamago Model SLPS-03534 | 2003 | KID Kindle Imagine Develop |
| Arcade Video game | Out Run 2 | 2003 | SEGA Corp. |
| Pachislot | Outlaw | 2003 | Eleco, Ltd. Aruze Group Japan |
| PC/MS-Windows CD | Outlaw Golf | 2003 | Simon & Schuster Interactive |
| Microsoft XBOX disc | Outlaw Golf - 9 More Holes of X-Mas | 2003 | Unknown |
| Nintendo GameCube disc | Outlaw Golf Model DOL-GOFP-EUR | 2003 | TDK Mediactive, Inc. |
| Microsoft XBOX disc | Outlaw Volleyball | 2003 | Unknown |
| Microsoft XBOX disc | Outlaw Volleyball - Red Hot | 2003 | Unknown |
| Sony PlayStation 2 disc | Over the Monochrome Rainbow featuring Shogo Hamada Model SLPM-65271 | 2003 | Sony Music Ent., Inc. Japan |
| Nintendo GBA cart. |  Overstorm | 2003 | Polyzoom Media |
| Video Slot Machine | OXO Reels | 2003 | Electrocoin Automatics, Ltd. London, England |
| Nintendo GBA cart. | Ozzy & Drix Model AGB-BOZE-USA | 2003 | Midway Home Ent. |
| Nintendo GameCube disc | P.N.03 Model DOL-GPNE-USA | 2003 | Capcom Ent., Inc. USA 13388 |
| Nintendo GameCube disc | P.N.03 Model DOL-GPNJ-JPN | 2003 | Capcom Co., Ltd. Japan |
| Nintendo GameCube disc | P.N.03 Model DOL-GPNP-EUR | 2003 | CE Europe, Ltd. Capcom Europe London, England 055060 |
| Redemption game (mechanical) | Pac-Man Ball | 2003 | NAMCO, Ltd. Japan |
