I don't understand OSR. If I felt the need to go back to the time before game design, I would just play B/X (which I have done). But aside from that RPGPundit is kind of an obnoxious polisperg who is to blame for many of the bad elements of 5e's design (as he himself will often gloat) so he is on my shitlist for many reasons.
No one understands OSR because it covers so much ground.
DCC (released by Goodman Games who now sucks tranny dick) is a high effort OSR that claims the OSR due to 70s aesthetics and their massive rule tome having typesetting straight from 1970.
OSE is literally just B/X with formatting and some rewording of rules (and supplements that cater to the creator's mushroom fetish)
In between you have Knave, LotFP, and White Hack/Black Hack which are BX/AD&D compatible system extensions. Basically Pathfinder to B/X's 3.5e. Because B/X is so simple, its very easy to rewrite and extend so you see a lot of low-effort.
You also have games like Maze Rats that use different dice.
You also see games that are just Settings; mechanically B/X but offer new classes, tables, etc. Again, given the slimness of the B/X rules its not that hard for your setting document to exceed the size of the OG rules.
There's also a bunch of "OSR Mods" to 5e like Torches in the Dark or even GMG's 5e Classic Modules conversions could be considered OSR.
From my personal observation/perspective, OSR has following (or at least attempts to accomplish these things)
1) Native or near-native compatibility with any D&D 1e/BX module released under TSR. Even if they don't use the exact Five Saves, they either say what they've been renamed or provide a "This save maps to this value" simple conversion. Monsters are tracked with HD.
2) Lower PC survivability; a step away from 3e's "superheroes". not just save-or-die effects. But Even high level characters are vulnerable if overwhelmed and character death is to be expected.
3) "70s aesthetic" - an appeal to nostaligia via art and theme. This is very problematic now, so you see a lot of gooners (Venger Satanis) trying to hide their perversions behind "Its Gonzo 70s Heavy Metal".
4) Combat that is quick, deadly and focuses more on strategy than tactics. Its not about positioning minis and using measuring tape/counting squares to figure out exactly how far you can get, its more "I run to the cluster of rocks over there". I'd probably include "enemies have morale" on there, but I think that'd getting into hair-splitting territory.
5) High degree of randomness, from encounter tables to rolled stats, to random spell effects.
6) Lack of skills/skill points/skill checks. I'm somewhat hesitant to include this one because even B/X rogue has
skill THEY'RE CALLED ABILITIES, MOM GAWD and most OSE systems include a list of things that classes are good at even if its very general. but its not to a PF/3e system where you pick skills from a list and assign points. And there's usually no "DC10" checks, its usually percentile. And that leads us to
7) Movement away from "Keywords" and towards "General Terms". Its not OSE without rules lawyers slap fights over ambiguous language. Unlike (especially) 4e and 5e, there isn't a focus on keywords or stacking specific words to clarifiy how armor is classified or what is made of metal.
I don't really considering C&C OSR because its more in the vein of 3e. Same with Basic Fantasy. Though I guess with the march of time....3e is almost old enough to rent a car.