What are the differences between Holmes D&D and Original D&D (as opposed to the differences between Holmes, Moldvay, and Mentzer D&D)?
OD&D is a backronym for the original line of products released in 1974 for Dungeons and Dragons. The name is used to distinguish from the other releases that came out in the 70s and 80s.
The Holmes boxed set is also a post-hoc naming of the boxed rules set written by J. Eric Holmes for TSR in 1977. The Holmes set marks when D&D was divided into Advanced and Basic Games. The Holmes box was mostly self contained, not a product line like OD&D, or AD&D that followed.
[ODnD] Dungeons & Dragons is first published in January as three booklets shipping in a woodgrain-colored cardboard box: Men & Magic, Monsters & Treasure, and Underworld & Wilderness Adventures. […]
[ODnD] A second 1,000 set printing of Dungeons & Dragons sells out in just under six months, followed quickly by a third printing of 2,000, all sold before the new year.
[ODnD] The Dungeons & Dragons supplement Greyhawk specifies the Thief and Paladin classes; its sequel Blackmoor introduces the Monk and Assassin.
[ODnD] For a fourth printing of 5,000 copies, Dungeons & Dragons switches to the white box that will be used for future printings of the original game.
[ODnD] D&D supplements III and IV—Eldritch Wizardry and Gods, Demi-Gods, and Heroes—are introduced.
[Holmes Set] Dungeons & Dragons is divided into Basic and Advanced versions. The D&D Basic Set edited by J. Eric Holmes becomes the first Dungeons & Dragons boxed set to ship with dice included.
The rules differences are quite extensive, and sometimes subtle and surprising. But the biggest one is that the Holmes set, being "basic" D&D was limited to only third level.
Other notable differences:
- Holmes initiative used your Dex score. (I still want to use that today).
- Holmes Magic Missiles were not an auto hit.
- The Holmes' weapon damage were not very differentiated. Daggers were awesome.
- No Bend Bars / Lift Gates scores or other sub-attribute abilities.
There are others. Some more are listed here.
If you want to read further I can not recommend this book enough:
It's a great read of the history of D&D.
The Designers & Dragons series by Shannon Appelcline is also a good read. It goes decade by decade through the RPG industry.