Is the OSR nostalgia? If it's nostalgia, it seems to be mostly nostalgia for an era of gaming - the mid to late 70s - that most of the participants never themselves experienced.
I think the very early OSR or proto-OSR (Dragonsfoot, C&C, OSRIC) was based around 1e AD&D modules and module-based play (mostly competition modules), the era that began with Against the Giants and ended in the late 1980s. I think that nostalgia and simple continuity of play plays a big role there - a typical Dragonsfooter in 2002 probably was just playing the same way they had been doing in 1986.
But the more recent West Marches, Grognardia and post-Grognardia OSR seems much more about the sandbox hexcrawling and the megadungeon, styles already falling out of favour in the late 1970s and only ever experienced by a relatively small number of gamers at the time. If it's nostalgia, it's nostalgia for a mythic past, rarely nostalgia for our own remembered past.