OD&D + Chainmail

 I love this so much. It really makes OD&D so much more playable when you just use it as a campaign system for Chainmail. So while all battles would be fought with Chainmail, OD&D would be used to calculate overland campaign movement rates, fleeing success, terrain types, random encounters, treasure, and allow a method whereby your troops get stronger and become hero types.

I've seen a lot of attempts to meld the d20 combat system with Chainmail's Man-to-Man combat table. The two earliest attempts were from Gary Gygax himself, first in Supplement 1: Greyhawk and again in AD&D 1e. I like the 1e table and I use it when I play AD&D, but for OD&D I think it's better to ditch the d20 system completely and only use Chainmail's d6 dice pools and 2d6 system.

I only use the Man-to-Man table in the case of Hero vs Hero combat or in the rare instances where it applies to Hero vs Monster combat. For everything else I use the Mass Combat system detailed earlier in Chainmail, where it takes a minimum of 4 light footmen to damage 1 heavy horseman. That means that if the player only has 3 light footmen, the heavy horseman is functionally invincible against them and can damage them with impunity. This is the expected result when strong monsters, such as dragons, attack regular troops.

This system works best with large troop numbers, so it isn't well suited to underground dungeon crawls with a small group of characters. Which is fine, as it works really well for overland campaigning with large armies and mass combat. Raising and maintaining an army is a huge money sink in this game and justifies a constant need for treasure. In fact I find it interesting if the players immediately jump into overland campaigning on day 2 of their adventure, after having gotten just enough gold to raise their first contingent of soldiers.