Unified THACO Resolution

The 1st Edition AD&D Dungeon Master's Guide contains the seeds of a standard method to resolve actions. In the example adventure, a party member can try to catch a scroll case before it floats away. It's just a regular hit against AC 4. This has all the feel of an on-the-spot "ruling" enshrined by happenstance, but it may also hint at an underlying unified "check" mechanic in early D&D. These pages are clearly instructive, showing how to handle uncertain events out of combat. I will talk about how to develop this into a very organic OSR house rule later:

Page 96 (as a room description):

Page 99 (as a play-by-play of the same room):

They seem to have abandoned this subtlety by AD&D 2nd edition, the idea-space being filled up by roll-under ability score checks for things like non-weapon proficiencies. Pretend for a moment you do not want to follow down that road. Your OSR-type game might benefit from a unified mechanic, especially if it is basically invisible. It's so natural you can even just pretend it was an on-the-spot ruling.

The Unified Hit

This is just taking the hint from the DMG and sticking with to-hit against AC as a general unified "check." As this was only ever a hint, let us flesh it out a little. Gauge the AC of a task by considering how well armored it is: a normal task is like an unarmored target (AC 10), a very difficult one like a foe in full plate & shield (AC 0). If the task is easier maybe don't call for a roll, but you see where this is going. If you are using the roll-high variant in OSE instead of THACO, this is going to look just like a modern D&D skill check.

Undoubtedly, you are now raising your finger, opening your mouth, and closing your eyes, a well worn "well akshually..." on your lips. You've found a great and terrible flaw in this system: "But this means Fighters are better at things - even basic knowledge! Congratulations, you are very smart. Consider this though: Wizards are good at spells and such, but their weirdo brains are not wired for practical solutions. Half of their education is probably calling up Imps to do things for them, like fantasy ChatGPT. Clerics and Thieves are kind of average folk who specialize in their divine magic or criminal hobbies. It is the humble fighter who must deal with actual reality and live by their wits, such as they are. Plus, just let them have a thing. They need it.

Note that Fighters do not always have a definite advantage either, especially at low levels. For example, it is good to set the AC of athletic or stealth checks equal to the AC of the character performing them. As such, it is harder to sneak or jump around in armor. In the case of thieves, this ends up giving them a slight advantage over more heavily armored fighters despite a lower to-hit. Furthermore, Thief class abilities should kick in on a failed roll.

Modifiers

Adjustments like bonuses or penalties can come from a relevant Ability score and/or the situation. This can get weird with AD&D or earlier, as the bonuses are not standardized. Do the best you can, using the recommendations below as a guide:

• Advanced D&D: For INT, use the Possible Number of Additional Languages, which is higher than the others but who cares. For WIS, use the Magical Attack Adjustment. For DEX use the Reaction/Attacking adjustment. For CON use the HP Adjustment. CHA is a little more complicated because it is all percentiles (way to go Gary); just use the tens place of the percentile as the modifier (i.e. -25% is a -2).
• Basic D&D: For INT, use the number of added languages. For WIS, use the Saving Throw Adjustments. For DEX use the Missile Fire Adjustment. For CON use the HP Adjustment. CHA uses the Reaction Adjustment.
  

The Dread Spectre of Skills

A good half of you read this and immediately started working up a way to cobble together a skill system for D&D and AD&D. You're probably oozing with greasy little homebrew standbys like using a d12 instead of a d20 if unskilled, patching on advantage/disadvantage, using wizard/magic-user THACOs if unskilled, implementing the Turn Undead table somehow, working up a massive trad skill list...

...maybe cool down a little.

Instead, just leave it all be. If the character would be good at something, due to their background or whatever, change the outcomes instead of the check. Don't even have them roll if it is something they would know. This is completely intuitive and demands no changes to the game. It's already there.

Example

The important thing is that you can implement this without changing anything in the game. It's more of a ruling than a rule, as they love to say in these circles.

In game, it could look like this:

DM: Fleeing onto the docks, you see a small sailboat, bobbing in the waves.
Player: I jump in and sail off!

DM: Does your character have any experience with boats?

Player: no, desert tribesman.

DM: You're missing something important, but fairly obvious. Make a to-hit roll vs. AC 9, add your WIS save adjustment.

Player: a 7, I miss whatever it is.

DM: You start to paddle out but the rope pulls tight and you stop with a sickening jerk. The mob on the docks is gaining, your lead disappearing.

Player: Oh damn I forgot - I reach over and cut the rope!

DM: You paddle out just in time.

If the character had a nautical background of any sort, the DM would probably just skip the check and just remind them, or even better allow a Check to paddle out of the dock even quicker and raise sail like a champ. In any case, whether someone has a background in something rarely needs rigid definition. It should be obvious after playing the character for a while.

Feel free to chime in below with observations, experiences from those who have tried it this way before, etc.