This may come across as unintentionally glib, but someone (I apologize that I can't give proper citation, the name escapes me) once said that the system is "OSR" if it can run Keep on the Borderlands without massive conversion. I'm not sure if I agree with that wholeheartedly, but I think it's a useful jumping off point in terms of narrative/design goals and playstyle.
The other thing that I find most (not all) OSR games share is that gold=XP and monsters should be unique and fantastic. Very few OSR games I have seen run are based around fighting a horde of goblins, and then next time some more goblins, and then after that some orcs that are just bigger goblins. In fact, I think at one point, one of Raggi's submission guidelines for LotFP was your module should only have encounters with things that had never been published before. I'm paraphrasing but that was the general idea.