I've been thinking recently about what the "OSR" really means, at least to me.
Old School Renaissance
Old School Revival
Open Shared Rules
I think "Open Shared Rules" is closest to it's true meaning. No matter the rules you play with - OD&D, AD&D, LL, S&W, OSRIC, LotFP WF, ACKS, C&C, BFRPG AS&SH, C&T, DCC RPG and others too numerous to list - they all share a common DNA.
I can run ACKS with a S&W setting and adventures from BFRPG and LotFP WF without any advanced prep besides reading the material in advance.
I can run AD&D / OSRIC with a S&W megadungeon and crib rules from ACKS.
I can run S&W using a 3.5 version of an Old School setting using DCC RPG adventures with all conversions on the fly.
All of this wonderful stuff fits together with minimal fuss and maximum enjoyment. Sure, OSR is also a style of play. With the right group and tools, 3.5 can be played "Old School". Wasn't that the point of the old 3x DCC adventures?
Open... because of the wonders of the OGL
Shared... because they all share a common DNA
Rules... because without the rules, there is no need for dice... and we NEED dice.
So, what does OSR mean for you?