O5R: My Take

Central Mechanic: Roll 1d20 + all modifiers verses an ascending Difficulty Class. Natural 20s are Critical Successes. Natural 1s are Critical Failures.

We use 5e Advantage and Disadvantage: Advantage: roll twice and take the higher. Disadvantage: roll twice and take the lower.

Initiative as Shadow of the Demon Lord. Ranged attacks and Spell Casting are Long Actions.

Stats: 3d6 down the line

Strength

Dexterity

Constitution

Intelligence

Personality

Luck

Modifiers as 5e

Now you might be thinking: why change the stats? Well, Luck is honestly such a useful stat that I hate ever having to go without it, but I didn’t want to go over six stats.

Level 0: HP: 1d4+Con Mod, Occupation: DCC table or your occupation table of choice

Humans reroll a Random Stat. Elves reroll Dexterity. Dwarves reroll Constitution. Halflings reroll Luck (Charisma if you don't use Luck).

Saves: Are applied to the individual stats as in 5e. Different classes will be able to add their levels to two different Saves.

My general philosophy regarding this conversion is thus: first off this is for my use. If I was solely interested in making a conversion that would be easy for 5e players to understand, I would have kept Charisma and Wisdom and not added Luck. If it is your endeavor to give your 5e players an easy framework for transitioning to OSR play, then I would advise keeping the original stats. 

Second, I don't think my alteration of the stats is necessarily as important as maintaining ascending ACs and Crits. These are honestly really important in my mind. A lot of what players in my experience, including my own time as a player, have a hard time understanding is when you change the way the game fundamentally deals with the dice. The omnipotent d20 with ascending DC is at the crux of everything in 5e. Truthfully, I think that is a great design choice. It allows the players to really easily recognize what the roll of the d20 means. The Nat 20 with all its cultural weight is a hard thing for a player to simply undo in their mind. It is hard for me to not look at a rolled 20 and not immediately begin celebrating.

Thirdly, I still think Advantage and Disadvantage is one of the smoothest and simplest ways of assigning bonuses that have yet to be invented. It is so easy to use and remember that I use it for almost every game I run. 

Classes are yet to come. I am still thinking about exactly how I ought to make them work without adding in all of 5e's bulk. I also think I want to avoid races as classes because I think that will be a hard thing for 5e players to come to grips with. I thought it was weird when I first heard of the concept. I started with 4e, after all. I have had a long journey to understanding OSR play and I still don't think I have the complete hang of it.