| Nintendo DS cart. |  Oideyo Doubutsu no Mori | 2005 | Nintendo Co., Ltd. Japan |
| Pachislot | Okinawa Storm | 2005 | Net Corp. Japan |
| Slot Machine | Old Glory Deluxe | 2005 | Cadillac Jack Duluth, GA, USA |
| Slot Machine | Oliver's Bar | 2005 | AGI Austrian Gaming Industries Novomatic Group |
| Slot Machine | Omega | 2005 | Bally Wulff |
| Nokia N-Gage card | ONE | 2005 | Nokia |
| Medal video game | One More Time Pro | 2005 | Unknown Korea |
| Nintendo GBA cart. | One Piece - Dragon Dream! Model AGB-BIPJ-JPN | 2005 | Bandai Co., Ltd. Japan |
| Nintendo GameCube disc |  One Piece - Grand Battle Model DOL-GOPE-USA | 2005 | Bandai America, Inc. 45557 |
| Nintendo GameCube disc | One Piece - Grand Battle! Rush | 2005 | Bandai Co., Ltd. Japan |
| Nintendo GameCube disc | One Piece - Pirates' Carnival Model DOL-GIPJ-JPN | 2005 | Bandai Co., Ltd. Japan |
| Nintendo GBA cart. | One Piece Model AGB-BONE-USA | 2005 | Bandai America, Inc. 45557 |
| Slot Machine | Onihama Bakusou Gurentai | 2005 | Ginza Japan |
| Pachislot | Onimusha 3 | 2005 | Rodeo Corp. Japan |
| PC/MS-Windows CD | Onimusha 3 - Demon Siege Model 683100 | 2005 | Ubisoft Ent. Montreuil sous Bois, France |
| PC/MS-Windows CD | Onnakyoushi Kimishima Misa | 2005 | Black Lilith |
| Pachinko | Onyanko Club | 2005 | Sansei R&D Japan |
| Pachinko | Ooumi Monogatari - Great Sea Story | 2005 | Sanyo Japan |
| Slot Machine | Opal Ring | 2005 | WMS Gaming, Inc. Chicago, IL, USA |
| PC/MS-Windows CD | Oppai Slider | 2005 | Illusion Japan |
| PC/MS-Windows CD | Oppai Slider 2 | 2005 | Illusion Japan |
| Slot Machine | Orchid Magic | 2005 | Aristocrat Leisure Industries Pty, Ltd. |
| Slot Machine | Oregon Gold | 2005 | IGT International Game Technologies Reno, NV, USA |
| Nintendo GBA cart. | Oshare Princess 5 Model AGB-BO5J-JPN | 2005 | Culture Brain |
| Pachislot | Osomatsukun | 2005 | Daiichi Denshi |
| Pachislot | Ossu! Bancho | 2005 | Daito-Giken, Inc. Japan |
| Redemption mechanical game | Ostrich Emotion | 2005 | Hope Japan |
| Nintendo DS cart. | Osu! Tatakae! Ouendan Model NTR-AOSJ-JPN | 2005 | Nintendo Co., Ltd. Japan |
| Taito G-Net card | Otenami Haiken Final | 2005 | Success Japan |
| Sinclair ZX Spectrum cass. | Othello 2005a Hb | 2005 | NyitraiSOFT |
| Sinclair ZX Spectrum cass. | Otodolo 2 Hb | 2005 | NyitraiSOFT |
| PC/MS-Windows CD | Otoko no Baken-jutsu Zipang Starter Pack | 2005 | MediaKite Japan |
| Nintendo DS cart. | Otona no DS Golf Model NTR-AGFJ-JPN | 2005 | Nintendo Co., Ltd. Japan |
| Nintendo GBA cart. | Ottifanten Pinball Model AGB-BOFD-NOE | 2005 | Trendverlag |
| Video Slot Machine | Outback Jack | 2005 | Aristocrat Leisure Industries Pty, Ltd. |
| Slot Machine | Outrageous Ollie's Oystermania | 2005 | IGT International Game Technologies Reno, NV, USA |
| PC/MS-Windows DVD | Oyaji no Chousen Mite Kitaeru Otoko no Nou tore - Kiso-hen Model MKO-027 | 2005 | MediaKite Japan |
| PC/MS-Windows CD | Oyaji no Chousen Otoko no Unou Drill | 2005 | MediaKite Japan |
| PC/MS-Windows CD | Oyaji no Katsuryoku Otoko no Kanji Doujou Shokyuu-hen Model MKO-030 | 2005 | MediaKite Japan |
| PC/MS-Windows CD | Oyaji no Katsuryoku Otoko no Sokudoku-jutsu 2 Asameshimae ni Shinbun o Kandoku | 2005 | MediaKite Japan |
