New OD&D Supplement — A Supplement for Perilous Realms

Basically just someone's 0E houserules, but worth a look, the PDF is free.

One thing, it introduces the attack method used in Sine Nomine's games, where you roll a d20, add an attack bonus, and then the armor class of the opponent (descending) and if it's 20 or more, you hit.

With the attack bonus being (like for monsters) simply the character's hit dice.