OSR — это, в первую очередь, набор принципов, к системе привязанный очень косвенно.
OSR game = based on old D&D rules. The OSR is specific; it is not a full-service revival of all things old-school. So any game which is roughly compatible with those old rules, and with existing OSR games, is itself an OSR game. You can change a lot and still fit within that framework.