Like OD&D being the grandpa of D&D, some things are not debatable. Castles & Crusades created the OSR. It was so forward thinking it jumped past First Generation clone status and was the first Second Generation Clone years before anyone else even had the idea.
That doesn't mean it's not without massive, massive flaws. But nobody can take Grandpa status from it, and nobody should be spreading idiotic ideas like "OSRIC was the first OSR game". I also read the official D&D Virtual Shitstory book where they were gracious enough to give the OSR an entire paragraph on one page, and erroneously pretended the OSR was a reaction to 4E because blaming things on 4E is one way to revise history and keep 3E in the best possible light so maybe you can get back some shitty Pathfinder players with 5E. Well none of that is the truth, because publication dates are a thing that exist, and some of us were around for the inception of the OSR.
3.0 came out in 2000
3.5 came out in 2003
Castles & Crusades came out in 2004
OSRIC came out in 2006
D&D4E came out in 2008
So the false first OSR game came out years before 4E, and the actual first OSR game came out seemingly as a reaction to D&D3.5. The entire movement was built with the same idea C&C had - take the bones of the new and the best of the old and meld it together. Considering how long it takes to put games together it's likely C&C was a reaction to 3.0 and just happened to come out a year after 3.5 started eating it's own.
Now I know I sorta dropped the entire central focus of the OSR nonchalantly in the last paragraph, and some seem to debate this as well - it's simply the case that originally the OSR was about producing new content for old games and quickly raced past that rather meager concept to become something more - a practice of Doctor Who designing D&D using the entire timeline as a basis rather than expecting/believing that an evolution took place. Things have been lost and gained with each edition. Some prefer fucking Attack Matrix tables and I can't even fathom that. But that person can make a game where they are highlighted. And then keep it far, far away from me.
So in essence, taking personal feelings out of it, games like Dungeon World, Neoclassical Geek Revival, Pathfinder, and Torchbearer which aren't fully sanctioned by the average asshole playing AD&D1E forty years later are explicitly within the OSR umbrella. And I say that as someone who hates Vinny Baker's guts and think Paizo are fucking the corpse of 3E into oblivion. But I'll save those rants for another time.
EDIT: Since this posting somebody has told me that Hackmaster might be grandpa, but as it existed not as part of the OGL but as a deal WotC made with Kenzer I don't consider it in contention. Plus it was a joke game never intended to be played regardless of how many are actually playing it now (which I assume is less than 10 groups worldwide)
