| Company | Based in | Founded | Industry |
|---|---|---|---|
| O.T.K Games | Beijing | June 2014 | video game industry |
| O3 Entertainment | United States | 2004 | video game industry |
| Oasis Games | — | 2011 | video game industry |
| Oddboy | Auckland | 2016 | video game industry |
| Odyssey Interactive | Waterloo | — | video game industry |
| Offbrand Games | — | June 7, 2024 | — |
| Ogre Pixel | — | — | video game industry |
| Oink Games | — | — | video game industry |
| Oizumi Amuzio | — | — | — |
| Okomotive | — | — | video game industry |
| OLM, Inc. | Setagaya | June 1994 | video game industry |
| One More Level | Kraków | 2014 | video game industry |
| One O One Games | Rome | 2019 | video game industry |
| Ongakukan | Tokyo | September 1985 | — |
| Onion Games | — | — | video game industry |
| Onion Soup Interactive | — | — | — |
| OnSkull Games | Thessaloniki | — | video game industry |
| Onteca (United Kingdom) | Liverpool | 2001 | — |
| OperaHouse | Chiyoda | April 2002 | video game industry |
| Oracle Corporation | Austin | June 16, 1977 | software development |
| Origin Systems | Austin | 1983 | video game industry |
| Orthrus Studios | Dundee | 2017 | — |
| Orube Game Studio | — | — | video game industry |
| Osmotic Studios | Hamburg | — | video game industry |
| OSome Studio | Lyon | April 10, 2013 | computer programming |
| Ouat Entertainment | Angoulême | 1999 | video game industry |
| Out of the Park Developments | — | — | video game industry |
| Outerloop Games | Bothell | — | video game industry |
| Outrage Games | Ann Arbor | 1997 | video game industry |
| Outright Games | — | 2016 | — |
| Over The Moon | Vancouver | 2013 | video game industry |
| Over the Top Games | Madrid | 2008 | video game industry |
| Overdose | — | — | — |
| Oxford Softworks | Buckinghamshire | 1989 | video game industry |
| Oxygen Studios | Croydon | 2007 | — |