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- Off An Rpg and subliminal Thriller made using RPG Maker.
- One Night Trilogy is a series of three freeware survival horror games.
- Orbiter is a space flight simulator with an emphasis on realistic orbital mechanics.
- osu! is a freeware simulator of the rhythm video games Osu! Tatakae! Ouendan and Elite Beat Agents.
Read more about this topic: List Of Freeware Video Games