For what it's worth, my working guideline of OSR is "A character or monster made in an OSR game should be able to have a fight with a character or monster made in an old-school D&D game with minimal issues." In particular, both should use d20 to attack and track damage in hit points. Also an OSR game should use the same six ability scores as D&D, possibly renamed.
ETA: I don't mean that this should be everybody's definition. It's just the one I have in my head.