I normally steer clear of these discussions here as they don't really pertain to RBE - Backswords & Bucklers is really the only OSR game to receive regular mention - but it's something that I've addressed a number of times on different discussion forums, so please forgive me for indulging in a bit of meta today.
For my part, I make a distinction - and by no means does everyone agree with that distinction - between the O[ld] S[school] R[enaissance], which is publishing retro-clones of pre-3e D&D and associated adventure modules, settings, and supplements, and an o[ld] s[chool] r[enaissance], which is a general renewal of interest in playing games from roughly the first decade of the roleplaying game hobby.
Soon, gamers decided to clone the other editions, for a mix of reasons - sometimes they simply liked the other TSR-era versions of D&D better, sometimes as a vanity thing, in my experience. Soon there were clones for just about all of the different versions of the D&D rules published by TSR - most prominently Swords & Wizardry and Labyrinth Lord - yet most of them weren't 'true' clones, in that the author's editorial voice was present to one degree or another, from everything to rules interpretations to adding house rules.
This is where I think it starts to get confusing for some people, I think. That inclusion of editorial voice led some gamers down the same path taken with Tékumel and Arduin, of making what was essentially campaign-specific versions of D&D. It was this 'wave' of the OSR that produced titles like AS&SH, ACKS, and LotFP as well as Carcosa.
Finally, there were those who decided to take the original rules and concepts in different directions - the OSR equivalent of Marvel's 'What If?' series - and thus they released 'clones' like Stars Without Number for sci-fi space adventures, Mutant Futures for post-apocalypse campaigns, and Flying Swordsmen for wuxia martial arts adventures.
One of the important features of all of these games is that, because they draw from the same root stock - D&D, and more specifically some variant of the SRD - they are often highly interchangeable with one another.
That, in a nutshell, is what the capital-O, capital-S, capital-R OSR is about.
At the same time this was occurring, and with some degree of mutual feedback and support, other gamers were noticing older games were still just as playable and just as fun as the stuff they could readily find on gaming-store shelves. Games like TFT and Chivalry & Sorcery popped up in discussion on rpg forums, as did Boot Hill or Champions, and with the help of second-hand retailers and eBay, they started finding their way into gamers' hands, and onto their tables, once again.
A few games never really went away. Fans of original, 'classic' Traveller were quietly humming along for years; their reprints were still available, supported by a ton of fan-created content that Marc Miller didn't attempt to stamp out the way TSR did. Call of Cthulhu and Pendragon received new editions, but remained close to their roots. GURPS was always there. Closer to home, Flashing Blades remained in print and En Garde and Maelstrom were republished, the latter with a new companion volume as well as adventures and supplements.
The presence of these games in online discussions began to grow. Awareness of availability went up. This was an old-school renaissance of a different sort, less publishing oriented and not fixated on D&D, but rather on digging up and playing older roleplaying games, discovering that once the dust was cleared away, they still held their shine. This lower case-o, lower case-s, lower case-r osr is also producing some clones of its own, like Legends of the Ancient World and Heroes & Other Worlds for TFT, FASERIP for MSH, and Classified for the James Bond RPG, but these are not OSR, which is a publishing effort specific to TSR-era D&D.
There are edge cases, as in all things. Is Castles & Crusades the first OSR retro-clone? Some say yes, others - including me - no. Is DCCRPG OSR? I don't know enough about it to have an opinion, but I know it's been argued both ways as well.
If I understand him correctly, Erik seems to argue that the OSR is the tent under which all these games fit. I think it's the other way 'round - that the OSR, the family of games with a shared D&D heritage, are part of a larger renaissance of interest in older roleplaying games. The OSR tends to get the most attention - or sucks the air out of the room, depending on your perspective - by virtue of the fact that it's derived from the World's Most Popular Roleplaying Game™, but we cannot forget the games that kept chug-chug-chugging along through the long White Wolf and Whizbros night to those which enjoyed a true renaissance after being long forgotten by all but a handful of stalwart fans.
I suppose where I most disagree with Erik is that 'all roleplaying games are D&D.' I certainly won't argue that's the perception from many outside the hobby; I don't know that there's any advantage to us accepting or adopting that label for ourselves, when it tends to maginalise the broad range of subjects, genres, and games of which the hobby is actually comprised. Erik argues that D&D is to roleplaying games what Xerox is to photocopying or Kleenex is to tissue. I believe that's self-diminishing. I would say that D&D is more like what poker is to card games: arguably the best known and probably the most visible, but nothing like the only game in town.