It seems like today is the day to define what "OSR" truly means.
Who am I to fail to play the latest round of the game?
OSR - Old Shit Rules!
If it it looks like, feels like, plays like and smells like old shit, it's OSR.
"Shit" in this context is defined as: When something is really cool, awesome, or in someways really interesting.
Sure, you could go here or here for other definitions of the day, but by tomorrow, they'll be obsolete. Old shit will always rule ;)