Tenkar posted a blog about this review of D&D 5E which in the course of being written tried to define the OSR, albeit from a relatively uninformed outsider's perspective, and the review included a throw-away line toward the end which said that D&D 5E proved the OSR didn't exist. Naturally the OSR rose up like a vengeful revenant and engaged in righteous anger to correct this issue. Including me, I admit! But the author of the piece, Andrew Girdwood, did respond to me and others and was basically clarifying/backing off on assertions that I gather were not taken as intended. In the course of my responding, I accidentally defined the OSR (from my perspective) so in the timeless tradition of OSR and hybrid blogs like my own I decided to immortalize that definition here:
I think the OSR community is best defined as a group of people with a common interest in the style and methodology of role-playing games exhibited in the formative years of the hobby. It covers a lot more than D&D in this sense (I am a big fan of Chaosium systems such as Call of Cthulhu and Runequest, for example, both of which are archetypal examples of OSR gaming from back in the day). I think attempts to define the OSR in the context of game design fail because there was no consistency in design back in the day, either; but there definitely was --and is-- a community that appreciates the "feel" of that era and replicates it with contemporary retroclones and new systems (edit: as well as the original games). About the only certainty I can say is that game systems and styles which focus on more progressive mechanics such as FATE are definitely not typical of the OSR methodology, chiefly because these more story-based/collaborative systems did not exist in any form back in the seventies and early eighties.
I think the only way to define the OSR is for EVERYONE to write down their short definition of the OSR, then someone like Dyverse collates it somewhere forevermore and fires a print copy into the Sun. Then and only then can the topic rest easy, for the Sun is the OSR definition's only weakness, unless you also happen to have a +5 vorpal long sword. That'll work too.