OED (Original Edition Delta) is a short set of house rules for Original D&D (1974 edition).
Many of us find the original game the purest, most playable, most DM-friendly version, but it does have some gaps and confusing points. OED clarifies and streamlines various topics to make these rules more approachable to the modern gamer.
The OED rules supplement is free for download at OED Games (www.oedgames.com). And don’t forget, scans of the Original D&D core rules can be bought at RPGNow.com (search for “Original Edition”).