This thread has evolved to be quite long by now, so I didn't have enough patience to read through all of it.
Anyhow...I started playing in 1981 with AD&D 1e and went on sabatical (1st child born) in 1987. By 87 I'd played AD&D1e+WSG, Gamma World and Traveller. While Original D&D was in my university gaming club's library, no one ran or played it by the time I joined in 83. As well, D&D variants like B/X or BECMI never got traction in my city, as LGSs were only interested in shelving AD&D as that was what the President (Gygax) of TSR was backing/writing. I didn't return to TTRPG play until the mid 2000s when 3e was the predominant edition.
To me the OSR acronymn can be read 3 ways, with each having different meaning - Old School Roleplaying, Old School Revival, Old School Renaissance.
For me Old School Roleplaying describes a common style of play that seemed to end somwhere around the mid 80's. Most folks in my community college club and private group were running the prewritten modules from TSR. For those there seemed to be a focus on adventuring, dungeon delving and treasure hunting, with a high degree of lethality. It seemed to be around 83/84 (long time ago so hard to be sure) when I was playing in my university club, that the TSR modules changed to what I'd call a more heroic style of play; i.e. more focus on narrative and PC survivability was better. At my university club and later comic shop tables, that change of style seemed to trigger a homebrew initiative. I.e. some DM's started exploring different/house rules, while others started creating highly narrative adventures/campaigns that went beyond the narrative being presented in TSR's.
Whereas Old School Revival to me best describes TTRPGs like OSRIC, Old School Essentials and Lamentation of the Flame Princess, where the authors are reviving an existing edtion, by clarifying the rules, providing good presentation and adding some options.
For me Old School Renaissance is more like Dungeon Crawl Classics. For it (and similar TTRPGs) the authors seem to be creating a hybrid that combine the OS Roleplaying style that I experienced in the early 80s, with rules and play mechanics from newer RPGs; i.e. 3e like DCs, skill bonuses and saves. Castles and Crusaders seems to be taking a similar approach, but I don't have much play time with it, so hard to be sure.
I don't know if that all makes sense, but based on my early experiences and more recent experiences, those are the ways that OSR could be described.