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- Obsidian Studios - Role-playing accessories
- Old Glory miniatures - Miniatures
- Old Kingdom Games - Role-playing games
- Omega Games - Board games
- On Demand Games - Board games
- One Small Step - Board games, card games, wargames
- On The Lamb Games - Miniature Games, Board Games, Roleplaying Games
- Operational Studies Group - Wargames
- Optimus Design Systems - Role-playing games (out of business; see SSDC, Inc.)
- Orisek Industries - Dice and supplies
- Osprey Publishing - Role-playing accessories
- Other Hands - Role-playing games
- OtherWorld Creations, Inc. - Role-playing accessories
- Ottoman Litho - Board games
- Out of the Box Publishing - Board and card games
- Outpost Wargame Services - Miniatures games
- Outset Media - Board and card games
Read more about this topic: List Of Game Manufacturers