Pro AD&D 1E aligned to d20 SRD
OSRIC is based on the well-known 1st edition of Advanced Dungeons & Dragons, arguably the most popular RPG of all time. It doesn't stick to is slavishly, though, but instead brings in a few innovations from the later "d20 SRD" to regularize the system a bit and fill in some of the gaps. It is, however, still very much a 1980s system.
Con Complex and inconsistent rules
There is no "general system" for doing things in the game - everything is a special case. This can result in a lot of page-flipping to find the particular rules for something, and in problems for new GMs who don't yet know the system well, or who aren't comfortable with improvising.