OD&D? Pah! The REAL Fantasy game is Chainmail. And it is way ahead of its time. Here’s why.
Chainmail has Dice Pools. When you attack some Light Foot with your Medium Horse, you roll 2d6 per horseman, and you get a success (kill) on a 5 or a 6. The dice pool mechanic wouldn’t be seen again until Shadowrun in ’89.
Chainmail has ascending Armor Class. Sort of. Chainmail man-to-man combat is run by crossindexing things on matrixes. On the melee table, there are headings for the different types of armor (No Armor through Plate Mail and Shield). On the Missile Fire table, the armor types are replaced with ascending numbers: 1 for No Armor, 2 for Leather, up to 8 for Plate Armor and Shield.
Chainmail has at-will spells. There are no spellpoints or rules for Vancian casting in Chainmail. A wizard can throw a fireball once a turn, if he likes.
Chainmail has rules for counterspells – and they’re simple: when an enemy wizard casts a spell, roll a target number on 2d6 to counter it. D&D 3e had counterspell rules that no one ever used because they involved readying an action. I don’t think any other edition has counterspells as part of the core rules.
Chainmail has rules for spell failure. A weak wizard (a seer) can try to cast a difficult spell – they just have a chance of failure. This was taken out of D&D, and generations of fans have tried to houserule it back in.