OD&D eventually had five original supplements, and recently with the advent of Swords & Wizardry, more and more folks are producing new OD&D supplements. For purposes of this blog, and unless otherwise noted later, I consider OD&D to consist of Chainmail, the original 3 brown books (hereafter LBBs), the five supplements (Greyhawk, Blackmoor, Eldritch Wizardry, Gods, Demi-Gods, and Heroes, and Swords & Spells), as well as the Carcosa supplement that has recently been released. In addition I will be drawing some inspiration and information from the first edition Deities and Demigods, only because it has more complete information on more pantheons than GDG&H and because in some ways I like its presentation of the Cthulhu Mythos better than that in Carcosa, though I will draw from "C" as well.
So, given these sources, here are the rules I am allowing. I posted these previously over on the OD&D forums. These are also subject to change as I go.
* Combat will use Chainmail, and be adjudicated using my rules interpretations found here. This will allow me to easily switch between man combat and mass combat, and I think using Chainmail gives OD&D a very distinct flavor.
* All character classes are allowed, including Fighting Man, Cleric, Magic User, Paladin, Thief, Assassin, Monk, and Druid. As Paladin is the only one of these to not have its own XP table, Paladins will require 10% per level more XP to advance than normal Fighting Men.
* Thieves will be per Philotomy's alternate class.
* No psionics, though this might change to a modified version of the psionics found in Carcosa (set powers instead of them changing every day, and Psionic will become a class, if I do this).
* Ability bonuses from Greyhawk are in, but only Fighters get to apply Strength and Dex bonuses in combat, and these bonuses must be split amongst all of the Fighter's die pool or attacks.
* Hobbits, not Halflings. Hobbits can go to 5th level in Fighting Man, not 4th.
* Dwarves can progress to 9th level as Clerics, 9th level as Fighting Men, and can multiclass with Fighting Man.
* Elves are unlimited as Magic Users.
* Extrapolated from M&T, ALL magic swords are Intelligent. No "normal" magic swords. Thinking of taking a page from Fred Saberhagen on this, and having only 21 magic swords in the world--7 dedicated to Good, 7 to Neutrality, and 7 to Evil. They were left scattered and forgotten after the First Great Godswar.
* Gods: Considering a pantheon constructed from Celtic, Finnish, Babylonian/Sumerian, and the Cthulhu Mythos. The former three are from Gods, Demi-Gods and Heroes, the latter ones drawn from first edition Deities and Demigods and Carcosa.
* Alignment: I have never been a fan of "Law" and "Chaos"--I'm substituting Good and Evil.