O3 Entertainment
Oberon Games
Oberon Media
Obsidian Entertainment
Ocean of America
Ocean Software
Odesta
O-Games
Omerta
Omninex
Omnitrend Software
On-line Systems
OPENetwork
Origin Systems
Ossian Studios
OUAT Entertainment
Outerlight Ltd
Outside The Box Software
Ozark Software
Note: bolded names indicate companies that released a game within the last twelve months.