Just what is OSR? There's a lot of different meanings to that, depending on who you ask.
But since this is my blog, I personally interpret it rather narrowly - material that is directly compatible with old school D&D and its offshoots, which includes Gamma World, Metamorphosis Alpha, and even Buck Rogers
Some games, notably Castles and Crusades and Mazes and Monsters, are close. Real close. You could probably use their materials with minimal conversion.
Others, like Dungeon Crawl Classics and the various ____ Hacks to me not only miss the point of old school games, they also differ enough in terms of mechanics that they don't interest me or outright annoy me.