I played OSR games and have spent 30 years running from them 
However I am intrigued by the revival - it is part of the scene I have not really looked at.
OK, those are three big questions, but Iâll do my best.
1: Must-read OSR games.
Part of what makes the OSR work is that the rules are mostly interchangeable, they are all (OK, mostly) variations on TSR-era D&D, so there isnât any one game which is essential. There are plenty to consider though:
B/X Essentials is a recent re-write of the Moldvey/Cook Basic and Expert rules. It provides all the classic rules and gameplay with forty years of presentation improvements.
Into The Odd is D&D distilled down to itâs absolute core. Gameplay focuses on avoiding danger, the mechanics are only supposed to come into play when the playerâs tactics have failed and they need to engage in a fair fight or roll a save to avoid something nasty.
Whitehack is D&D with mechanics of improvising details like magic, species and setting during play through negotiation between the GM and players.
Beyond The Wall Is another Basic D&D varient but this one provides players with playbooks full of random tables to generate their charactersâ history. As they work through these they also generate the village they all live in and the people that live there. The GM then takes those NPCs and uses them in a scenario book to generate a random, bespoke, adventure.
The books which should really be considered essential are the settings and scenarios:
Yoon Suin is a complete fantasy setting themed around Tibet and Nepal. The whole setting though is detailed through random tables enabling you to generate as much as you need and every time it is used it will provide a different experience. I love using it and itâs wonderful to have a setting so far removed from medieval Europe.
Hot Springs Island is a systemless island hex-crawl with one book for the GM (The Dark of Hot Springs Island) and another, less accurate and detailed one for the players (A Field Guide to Hot Springs Island).
The Hill Cantons books. Fever Dreaming Marlinko, Slumbering Ursine Dunes, Misty Isles of the Eld and What Ho, Frog Demons. Four books detailing a weird Slavic land focusing on utility at the table and providing incredible places for playerâs to explore as they chose.
Michael Prescottâs mini-adventures. Free on his blog, each one of these is an incredible point of adventure for you to drop into your own games.
Dungeon Full of Monsters. Fifty small dungeons to randomly connect together into a megadungeon.
Deep Carbon Observatory. A grim adventure backward through recent and ancient tragedies. Unpredictable, horrifying and exciting.
There are many more than just those, but those are my favourites.
Also read David Perryâs Principia Apocrypha. This is essential.
2 What is OSR.
Well thatâs a discussion which has been going on for a while.
Anything which is based on TSR D&D.
Most would include Maze Rats and Far Away Land, WFRP and Tunnels & Trolls which are different systems but they play in much the same way.
Some people argue that other retro games like WEG Star Wars or Traveller or RuneQuest should be included. I kind of agree, but I find that the addition of skill systems and the removal of classes changes the dynamics quite a lot. These games can still be played in an OSR style, but itâs less natural.
Pretty much anything which gives players the ability to control the fiction beyond the remit of their own PC, or any shared narration will pull the game out of the OSR sphere. So Fate Point, Bennies and other meta-currencies. Players setting scenes. Any form of pre-arranged scene framing. This is because OSR games are pretty much all focused on seeing where the players take the story, through the actions of their characters. The GM provides the situation and then adjudicates the playerâs response. Repeat. It is strictly not about the story the GM want to see, or cool tropes the players suggest should occur. It has a lot in common with the PbtA mantra, âPlay to see what happens.â
The Principia Apocrypha explains this better.
3 How has the player/GM balance changed?
Not much really. Only that games are explicitly about what the PCs do, not what the GM wants to happen. GMs still control the setting and the NPCs, Players control the PCs, and maybe their (brief) backstory.
Hope that helps. Iâm happy to expand on anything.
Thanks so much 
It helps me to understand - I must find a Gauntlet game to sign up and try.
The two OSR games that I would recommend for seeing what can be cool and innovative about the genre include:
Torchbearer is an interesting game but I wouldnât call it OSR. Itâs too focused and play is very structured. The Burning Wheel family of games are kind of the antithesis of the OSRâs ârulings not rulesâ.
In addition to what Brian and lindevi suggested:
A) Perhaps a bit obvious, but the original 1974 Dungeons and Dragons white box, to see the original rules as written.
B) More of an oddball suggestion: Rules to the Game of Dungeon, a set of rules published in 1974 by a teenager who had played with people who had played in D&D co-creator Dave Arnesonâs group and didnât realize there was a published set of rules. Itâs very silly and diverges from D&D in fascinating ways that illustrate how differently the game could be interpreted even back then.
This is a huge can of worms and raises the age-old question of whether thereâs a difference between OSR (old school revival/renaissance/rules/&retroâŚ) and actual old school. Hardly anyone would argue that Traveler, Call of Cthulhu, or WEG Star Wars are not old school, but are they âOSRâ? Are actual TSR products OSR?
Stars Without Number author Kevin Crawford once defined OSR as, roughly, being compatible with TSR era D&D mechanics insofar as you could say about an OSR product âIf you get this thing, you can use it with a lot of things you already have.â For a while it seemed like this had become the most commonly accepted definition, but the acceptance of Troika (based on Fighting Fantasy) and Mothership (based on CoC I think?) as OSR has me questioning that.
Many others, meanwhile, think of OSR more as a playstyle, rather than a set of mechanics or particular games, that emphasizes lethality and player skill vs character skill. To me this further differentiates OSR from old school because I went to lengths to avoid lethality in my Holmes Basic Keep on the Borderland campaign. My friends and I were drama nerds, not wargamers, so we always tended more towards the narrative side of things. We made the game our own, with the second published version of the rules, with few pre-conceived notions of how to play, which to me is as old school as you can get, but we were doing it âwrongâ if you take the Principia Apocrypha as OSR gospel.
In my Holmes Basic game, I naturally gravitated towards giving the players more of a say in the game because it was more fun for us that way. Whatâs changed is that now there are games that specifically advise this approach and others that are more clear that the PCs shouldnât have input into anything other than their own charactersâ actions.
This is me & my group, at least to some extent. I get that thereâs a distinct set of mechanics and sort of design philosophy that underpins the OSR, but for our group, itâs really more about emulating a certain style of play that maybe isnât OSR per se but tends to be partnered with it. âHigh lethalityâ isnât necessarily even how Iâd describe it, though thatâs not far off, but a style of play that emphasizes that the adventurers are not necessarily special; theyâre not superheroes and theyâre squishy just like everyone else. They become heroes through play.
As a GM/DM/Judge/MC Iâm a bit too interested (and invested) in fiction-forward approaches, and I like rules (as well as rulings!) so I find myself being more drawn to OSR-adjacent games. @jasonlutesâ Freebooters on the Frontier hits that sweet spot for me, but itâs definitely a non-OSR game that plays with certain OSR conventions.
Of the more âstraightâ OSR games, Into the Odd is definitely the one thatâs most appealing to me. I havenât read it yet, but every time someone talks about it I get a little excited. It seems like a cool approach. Otherwise Iâve read OSRIC, Swords & Wizardry, Lamentations of the Flame Princess, and Maze Rats, and have some familiarity with B/X Essentials, and to be honest other than Maze Rats I find them kind of ⌠boring? But the appeal of OSR stuff to me has nothing to do with nostalgia for OD&D, and I have a hard time seeing OD&D as being worth emulating to that degree. That sounds harsher than I mean it to, but hopefully you get me.
Yeah, I probably didnât do justice to the style (Principia Apocrypha goes into more detail. That idea of not starting out as a superhero aspect is a super important aspect I didnât mention. But Iâve noticed a real obsession with lethality in the OSR community (from Deep Carbon: âIf you cannot kill at least one player with this giant then you are probably doing something wrong. Kill them. Make them afraid.
Explain nothing.â [I really hope he means *player character*âŚ])
Iâm totally on the same page about rulesets. Next time I run a dungeon crawl Iâll almost certainly reach for a PbtA game, but I still want to play Into the Odd some day.
Some interesting stuff happening in two I didnât see mentioned above: second editions of both The Index Card RPG and the Black Hack.
Oh yeah, definitely donât mean to say that the OSR community in general isnât obsessed with lethality, just that Iâve found a healthier way of looking at it 
To be fair, I have already had a PC die and not come back from it, and all of the PCs except our fighter have been at deathâs door. (And of course, Freebooters is far less forgiving of getting to 0 HP than stock DW is âŚ)
That idea that weâre supposed to be killing our PCs is a little bothersome to me. And I agree, hopefully, they meant PC! Though some DMs I know, I wouldnât be surprised to find out âŚ
The Index Card RPG has a central focus on the artifacts used in play (based on index cards, of course). It gives the game a kind of unique tone that I canât quite explain.
Judging totally as an outside observer, it seems like the Black Hack hits some kind of sweet spot for a broad swath, and there are lot of genre hacks (i.e. implementations of things like Gamma World, games with mechs, and such). I dunno. Never played it, but its seems like the activity around it is categorically different than around other retro-clones. If OSR is your thing (itâs not mine, particularly), it would be worth finding out why.