Labyrinth Lord - A retro-clone of the B/X D&D rule set.
LBB's - The Little Brown Books, a reference to the three booklets that made up 0E D&D. The parchment-colored, digest sized booklets are Men & Magic, Monsters & Treasure, and The Underworld and Wilderness Adventures.
LL - Labyrinth Lord
Megadungeon - A dungeon so large, and constantly changing, it could challenge a party from 1st level through to the endgame. While Gary Gygax and Dave Arneson both created megadungeons for their home campaigns (Castle Greyhawk and Castle Blackmoor), neither was ever published.
Microlite20 - A simplified, rules-lite version of the D20 system.
Microlite74 - A simplified, rules-lite version of the D20 system that recreates the feel and style of 0E D&D. The 74 in Microlite74 is a reference to the year 0E D&D was first published, 1974.
OA - Oriental Adventures
OD&D - Original Dungeons and Dragons, another name for 0E D&D. Sometimes the Holmes set is included.
OEC - Original Edition Characters, a supplement to Labyrinth Lord that provides rules for characters from 0E D&D in place of the default B/X D&D characters in Labyrinth Lord.
OGC - Open Game Content, any material release under the Open Gaming License (OGL).
OGL - Open Gaming License, an open-source license released by Wizards of the Coast in 2000. While it was commonly used with the D20STL up until 2003, it's now used mainly to publish retro-clones of previous editions of D&D.
OPD - One Page Dungeon
OSR - Old School Renaissance, a surge in popularity of previous editions of D&D. Common themes are are megadungeons, sandbox campaigns, and an emphasis on player rather than character skills. Matthew J. Finch's A Quick Primer for Old School Gaming articulates many of these concepts quite well.
OSRIC - Old School Reference & Index Compilation, a retro-clone of 1E AD&D.
47 minutes ago