OD&D: There Is No Substitute

You can now buy Original Dungeons & Dragons at D&D Classics. Specifically this is the 2013 reissue by Wizards of the Coast, which added new cover art to the OD&D booklets and cleaned up the typesetting (using the same Futura font).

At $10, you get the  three original  booklets in their final 7th printing form. This unfortunately removes the Balrog from the game, but you can find Zach Howard's Balrog Reference Sheet which includes the OD&D monster listing, the relevant rules from Chainmail, and all references to the Balrog that had been removed through the first four books of OD&D. So with that sheet you get back the original and best of the demonic beasts haunting the dungeons.

The rest of the references are just names. Ents became treants, hobbits became halflings, et cetera. There is no special need for a sheet, just have your players use the correct terms.

If you want a reference for the setting material implied in the booklets, I wrote The Original D&D Setting which is a modestly popular resource. Philotomy's Musings are a set of ponderings that you should read if you want to run OD&D, as they establish a good baseline set of items.

Should you want more monsters, there is a compilation here. I would recommend spending some quality time over at Finarvyn's Original D&D Discussion forum in general as it has lots of ideas for things you can do with OD&D.

There is no substituting for the original booklets. Read them; check out the supplemental material; read them sideways if you have to, but by all means, see what the original game had to say. And play it - for its simplicity as well as its richness.

Follow its procedures for dungeon stocking, and you'll find that the dungeons Gary was looking to create are very different from the ones most gamers are used to. Get into its simple exploration rules and you'll find the heart of how the game is meant to run. Run your encounters with its reaction tables and there's a whole social game that is so easily ignored. Construct combat on its basis and it quickly becomes clear that this is not a game for fair fights (or, unless you ignore morale, a game where every fight is to the death). This is a brilliant game. Enjoy it.

It is lightning in a bottle and while plenty of other games are enjoyable, nothing will ever substitute for the original work that started this hobby.