On his House Rules & Campaigns Forum, The Perilous Dreamer asked "What is a House Rule?".
The easy answer is "Any rules you decide to change", but this really doesn't place a limit on how far one might choose to distort the original rules. If you start with OD&D as written and end up with poker or ice hockey, did you still make a house rule? Of course not, the notion is ridiculous as there are limits on how much you can alter something and still be able to call it the same game.
So the question becomes "how much" instead of "how little" can be changed and still considered a house rule. So what core rules cannot be changed without a game no longer being recognizable as OD&D?
At its heart, OD&D is a game about exploring the underworld and uncharted wilderness. The essential rules of the game involve how characters move through a dungeon or outdoor map. Classes, equipment, magic, monsters, treasure, traps, and tricks can all be changed without changing the game. Pre-1974 wargames and post-TSR era D&D incorporate all those elements listed, but are each fundamentally different games than the D&D I play. However, a game of Candyland might very well be made into OD&D as long the pieces explore their board according to the procedures prescribed in The Underworld & Wilderness Adventures.