Convert 5e monsters to OSR standards

I can't remember what group I was in, but someone asked about converting 5e monsters to OSR/BFRPG. This is how -I- would do it. I usually work with S&W so this answer is specific to that, but all the OSR games are pretty close, so...

If you don't want to do this, that's fine! It's YOUR game, play it how you want!

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Only four things really matter in OSR: HD, AC, Saves, and Damage. Save(s) are HD-related, so we're down to three.

HD - Do NOT use as written. Compare Challenge and attack bonus. HD will be somewhere around there.
AAC - use as written +1 (since AC 10 is unarmored in 5e, vs AAC11 in S&W). 5e has a reduced AC range compared to 3e, so it works fine.
Damage - Damage is tricky. Very roughly, add the creature's damage dice together, then divide them by either 2 or 3. Divide those between the creature's attacks. Drop modifiers for now.

Special Attacks - In OSR, saves are based on the defender, not the attacker, so you don't need a number here. Maybe a bonus or penalty if it's particularly weak or strong. Divide damage by 2 or 3, as it pleases you.

Note - Advantage - The exact benefit of Advantage varies depending on the target (which is usually a save.) If you don't want to use advantage, consider it to be a +4 bonus/penalty to a save. If you want some variability, you could consider it a +3/-3 in attack/defense situations; a +4 to saves; and a 25% magic resistance. Or you could roll 1d4 and add that to the save or attack. Whatever pleases you.
Ability Scores - I keep the ability bonuses, but drop them by ONE, so a +1 is +0. Str can get added to damage if you feel like it, and possibly attack rolls. No bonuses over +3 unless it's truly epic. Con gets added ONCE to HD, so Con +3 would be 5d8+3 on a 5HD creature.
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Example: 5e Harpy has 7HD, AC 11, Challenge 1, a claw attack (+3/2d4+1) and a club (+3/1d4+1). The only notable stats are int -2 and dex +1, which adds to AC so we'll just pretend we're actually adding up ACs.

• 1 HD is probably too low, so 2-3HD is logical. 
• AC 11 is a AAC12, or 7 [12] in S&W format. 
• Damage added together is 3d4 . 
• OSR monsters tend to have multiple claw attacks, so 2 claws at 1d4 each and a club at 1d4. 
• A -1 to Int puts them in the "Low" range if that matters.

The actual S&W harpy has 3HD, AC 7[12], 2 talons at 1d3 each, and a "weapon" at 1d6. A S&W club actually does do 1d4, so either the harpies are using upgraded clubs or something else, like heavy maces or shortswords. The special abilities have the same overall result in both systems (harpy song charms and attracts victims), but 5e has more detail if you want that. Basically, nailed it.
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Example 2: 5e Annis has 12d10 HD; AC 17; Challenge 6, a bite (+8/3d6+5); 2 claws (+8/3d6+5), and a Crushing Hug that does 9d6+5/round if it grapples. Ability bonuses, subtracting 1, are Str +4, Con +1, Wis +1, Chr +1. The spell abilities (disguise self, fog cloud) are minor.

• Again, 12HD is too high, so we'll drop it to 7HD (right between Challenge and attack bonus). 
• AC 17= 1[18]. 
• Total damage dice are 9d6; annis are combat monsters so halving and rounding up makes sense - call it 2d6 per claw, 1d6 per bite. Even a strength bonus of +4 seems high though, so we'll go with +3 (2d6+3/2d6+3/1d6+3) 
• Crushing Hug would do 5d6+3. S&W doesn't have disguise self, change self, or alter self, so just use the 5e spell for its effects. Obscuring mist replaces fog cloud. 

Looking at Monstrosities, the canon S&W annis has 8 HD. AC is 1 [18]. Attacks are 2 claw attacks at 2d8 each, and a bite at 1d8. If they hit with both claws they can "hug and rend", hitting automatically with all 3 attacks each round thereafter (ie 5d8/round). We were a touch low on the HD. Damage-wise we were closer than it looks: 1d6+1 for these purposes averages to the same as 1d8, so we end up with 2d8+1/2d8+1/1d8+2 and 5d8-2. I'd call it a wash. (Rend is a frightening ability anyway; running a game back in the day I hit an NPC fighter/m-u with rend and she went from 75% of full hp (the initial attacks hadn't done too much) to something like -20hp instantly.) 

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Summary: 5e and the original OSR mechanics have more in common than a lot of people give them credit for. You can absolutely convert monsters from 5e to OSR standards with a pencil and a post-it note (post-it optional; I might just start writing notes in the margins). At the table, even!

Addendum: Look at existing monsters if you're not sure! If you are using S&W, there are monsters in the core rules, plus Monstrosities and Tome of Horrors Complete S&W edition. It's very possibly S&W has more monsters than any other OSR system already. The 1e MM, FF, and MM2 are the original source for many creatures. Blood & Treasure has two very full monster books that are OSR-compatible (B&T started off as a S&W/5e hybrid and I believe maintaining compatibility was a big factor - most of the monsters there can be played straight out of the book, and there are a LOT of monsters.) This isn't an exact science - relax and have fun with it!

Anyway, I hope this helps someone. Cheers!