I know O5R is seen as an oxymoron by many, but let’s go through the “Old School Principles for Players” section of Principia Apocrypha and determine what’s the least we have to change about 5e to live this philosophy. Its principles:

1. Learn When to Run
2. Combat as War, Not Sport
3. Don’t Be Limited by Your Character Sheet
4. Live Your Backstory
5. Power Is Earned, Heroism Proven
6. Scrutinize the World
7. Interrogate the Fiction
8. The Only Dead End Is Death
9. Let Your Creativity Flow
10. Play to Win, Savor Loss

I would argue most of the rule changes have to happen “behind the screen” rather than for the players. For the players, though:

1. Learn When to Run – In my O5R campaign, the players know that encounters aren’t balanced. The fear is so strong they now sometimes run from threats they can defeat.
2. Combat as War, Not Sport – They’re better than in our straight 5e campaign but have room to improve. After swearing they would hate a session without combat, we had a session without combat, and they all enjoyed it.
3. Don’t Be Limited by Your Character Sheet – This one has taken coaching. “I approach the leader and do a Persuasion check.” “OK, but first what are you saying?”
4. Live Your Backstory – Our Session 0 had them describe their relationship with the PC of the player next to them. (Next campaign I want to approach this with a DCC-style Level 0 funnel.)
5. Power Is Earned, Heroism Proven – They recognize this principle and are already planning ahead. “We’ll attack that goblin tribe after we’ve learnt more about their lair and progressed further. Maybe we can convince the Wolf Claw orcs to attack with us.”
6. Scrutinize the World – This is why we abandoned Forgotten Realms (the most well-documented campaign setting in history) in favor of a homebrew hex crawl. (Thanks to Hex Describe.) I still can’t get anyone to make maps, though. But they are deciphering the runic alphabet I’m using!
7. Interrogate the Fiction – This is the hardest for them. There is a retreat to mechanics. “I do a Perception check.” “First, what are you looking at and for?”
8. The Only Dead End Is Death – I’d argue removing resurrection-style spells is the main change needed to 5e. This group was resurrection-happy in Forgotten Realms. (Though Holmes wrote, “A seventh level cleric can raise the dead, if you can find one! Also, of course, wish rings and other magic can restore the deceased adventurer to his comrades and friends!”)
9. Let Your Creativity Flow – They often come up with very creative approaches to situations and problems. Last campaign (which wasn’t O5R) they used a soul-stealing sword to capture the soul of the lich, when they couldn’t find her phylactery.
10. Play to Win, Savor Loss – They do not savor loss. Three PCs were aged 10 to 40 years each due to a ghost’s Horrifying Visage and one of the players (25ish IRL) is so angry IRL that her character is now 38 years old that I’m starting to feel old indeed! But at least that has inspired an interesting quest to find the ingredients for a potion of lesser restoration (which will only roll back the clock for these three).

Behind the scenes, there’s a lot more work for the DM to use O5R:

1. Learn When to Run – The 5e DMG says, “Choose the starting attitude of a creature the adventurers are interacting with: friendly, indifferent, or hostile.” I use the Holmes 2d6 rules: 2, attacks; 3-5, hostile; 6-8, uncertain; 9-11, friendly; 12, enthusiastic.
2. Combat as War, Not Sport – They haven’t realized it yet, but every short rest I roll 2d6 and if a 6 comes up the rest is interrupted by a wandering monster. I’m trying to get them to better husband their resources.
3. Don’t Be Limited by Your Character Sheet – I’ve kind of given up here. I tried to enforce Adventurers League rules of PHB+1 but that doesn’t help the DM when everyone picks a different book. I have characters from XGE, from Eberron, from Unearthed Arcana. But this is old school, too; Holmes wrote, “At the Dungeon Master’s discretion a character can be anything his or her player wants him to be… Thus, an expedition might include, in addition to the four basic classes and races (human, elven, dwarven, halflingish), a centaur, a lawful werebear, and a Japanese Samurai fighting man.” (Which is why I always laugh when OSR purists insist on 4 races and 4 classes.) I am definitely regretting the artificer, though, as I lose some control over the magic-item economy.
4. Live Your Backstory – I’ve nudged the players out of town, as 5e had them expecting more stuff to happen in town, especially related to backstory.
5. Power Is Earned, Heroism Proven – I’ve custom-created a magic-item table that offers limited upgrades. They’re all about leveling up, though, so that’s how they see their “power earned.” (Depending on how the session went, I award a third to half the XP they need to level up. This group has no interest in spending loot on manors, retainers,  political influence, etc. I tried repeatedly last year.)
6. Scrutinize the World – The most work, by far. I spent a bit of the summer working on the campaign setting, so I wouldn’t have to prep much during the year. (Our last campaign ran from September to July.) It’s definitely cliché old school – they’re in one of the last bastions of human society, uncertain of what happened to the empire their city-state was part of 4 centuries ago or why their city-state was spared.
7. Interrogate the Fiction – I’ve tried to be better about giving each location three interesting features to interact with (a la Sly Flourish).
8. The Only Dead End Is Death – We use the three standard death saves. Too ingrained for the players to change, though we talked about it.
9. Let Your Creativity Flow – I’m adding constraints to encourage creativity. Our last campaign, in Melvaunt, often involved players buying whatever magic item they wanted (from Thentia). Every player had a bag of holding; no bags of holding in this world, and we’re tracking encumbrance.
10. Play to Win, Savor Loss – They really don’t want to start new characters at level 1 but want to introduce them at the same level as the dead character. This bums me out, but they were unanimous.

I have players that only know the core 5e mechanics of combat, attribute checks, and advantage/disadvantage, and do fine. And those mechanics are more than rich enough to hang an OSR game on. As for many of the other rules, they are often race and class and level specific, and those players who want to live off their 5e character sheet can and do. If I could, I’d limit the spell list more – many of the spells seem too powerful: my players use Mage Hand to pound the dungeon floor checking for traps, and use familiars to scout far ahead in the wilderness.

Still, my players want 5e and interesting sessions; I want OSR; so far O5R has made us all happy.

(I posted an earlier version of this on Reddit. Check out the comments.)