I've seen it more often as OD&D. I don't know for sure, but I can make a darn good guess gauging by context. OD&D stands for "Old Dungeons and Dragons", which would mean any version of D&D released earlier than 3.0. The A isn't required because only 2nd Edition ever used the term "Advanced"... so "Old Advanced Dungeons and Dragons" would be a release from the Department of Redundancy Department. AD&D refers to 2e only. OD&D refers to, as far as I can tell, 1e AND 2e. As I said, anything before 3e.