| O.T.K Games | People's Republic of China | June 2014 | 1 | — |
| O3 Entertainment | United States | 2004 | — | 1 |
| Oasis Games | People's Republic of China | 2011 | — | 2 |
| Oberon Media | United States | 2003 | 2 | 3 |
| Obsidian Entertainment | United States | June 12, 2003 | 10 | — |
| Ocean Software | United Kingdom | 1983 | 12 | 23 |
| Oddboy | New Zealand | 2016 | 1 | 1 |
| Oddworld Inhabitants | United States | 1994 | 6 | 2 |
| Odyssey Interactive | Canada | — | 1 | 1 |
| Offbrand Games | United States | June 7, 2024 | — | 1 |
| Office Create | Japan | April 3, 1990 | 1 | — |
| Offworld Industries | Canada | February 2015 | 1 | 2 |
| Ogre Pixel | Mexico | — | 1 | 1 |
| Oh, a Rock! Studios | United States | March 24, 2014 | 4 | 4 |
| Ohio Distinctive Software | United States | — | 3 | — |
| Ohsat Games | Germany | — | 1 | — |
| Oink Games | Japan | — | 1 | 1 |
| Oizumi Amuzio | Japan | — | — | 1 |
| Old Skull Games | France | January 20, 2015 | 4 | — |
| OLM, Inc. | Japan | June 1994 | 1 | — |
| Omega Force | Japan | 1996 | 25 | — |
| One Compath | Japan | January 20, 1997 | — | 1 |
| One More Level | Poland | 2014 | 3 | 1 |
| One O One Games | Italy | 2019 | 1 | — |
| One Up Plus | Australia | — | 1 | — |
| Ongakukan | Japan | September 1985 | 1 | 1 |
| Onion Games | Japan | — | 1 | 1 |
| OnSkull Games | Greece | — | 8 | 8 |
| OperaHouse | Japan | April 2002 | 1 | — |
| Oracle Corporation | United States | June 16, 1977 | — | — |
| Orange Pixel | Netherlands | — | 7 | 6 |
| Orbital Knight | Poland | 2012 | 2 | — |
| ORCA | Japan | — | 1 | — |
| Orcsoft | Japan | 2009 | 1 | — |
| Origamihero Games | Austria | — | 4 | 4 |
| Origin Systems | United States | 1983 | 21 | 10 |
| Orion Pictures | United States | 1978 | — | 1 |
| Orthrus Studios | United Kingdom | 2017 | 1 | — |
| Orube Game Studio | Brazil | — | 2 | 2 |
| Osgoode Media | United States | 2016 | 1 | 1 |
| Osmotic Studios | — | — | 2 | — |
| OSome Studio | France | April 10, 2013 | 3 | — |
| Ossian Studios | — | 2003 | 2 | — |
| Other Ocean | United States | 2006 | 7 | — |
| Other Tales Interactive | Denmark | — | 1 | 1 |
| Otomate | Japan | April 10, 2007 | 16 | — |
| Ouat Entertainment | France | 1999 | 4 | — |
| Out of the Blue | Spain | — | 1 | — |
| Out of the Park Developments | Germany | — | 19 | 19 |
| Outerlight | United Kingdom | December 2002 | 1 | — |
| Outerloop Games | United States | — | 1 | — |
| Outfit7 | Slovenia | 2009 | 7 | 1 |
| Outrage Games | United States | 1997 | 1 | — |
| Outright Games | United Kingdom | 2016 | — | 29 |
| Over The Moon | Canada | 2013 | 3 | 2 |
| Over the Top Games | Spain | 2008 | 1 | 1 |
| OverBorder Studio | Taiwan | — | 1 | — |
| Overdose | Japan | — | 1 | 1 |
| Overflow | Japan | 1997 | 4 | 1 |
| OverGamez | Russia | — | — | 4 |
| Overkill Software | Sweden | 2009 | 3 | — |
| Oversight Productions | United Kingdom | August 28, 2019 | 1 | — |
| Oxford Softworks | — | 1989 | 2 | 1 |
| Oxygen Games | United Kingdom | 2004 | — | 5 |