1. Companies starting with O:

      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.