Oriental Adventures is a term on /tg/ that can refer to either of two things.
Firstly, it can refer to a specific set of Setting Aesthetics, where the "default" neo-Medieval European trappings of traditional fantasy are removed and replaced with those of Asian or "Oriental" fantasy. Most consider this aesthetic synonymous with Japanese fantasy, but it can also refer to Chinese, Korean, Indian, or a mashup of two or more of these regions - the Japanese/Chinese/Korean mashup is particularly common, since a lot of real-world fantasy elements are actually shared between those three countries in real life.
Secondly, it can refer to either of the two Dungeons & Dragons sourcebooks created to provide advice on running D&D with this setting aesthetic. The first version was written by Gary Gygax himself for Advanced Dungeons & Dragons 1st edition, whilst the second was written by Wizards of the Coast for 3rd edition. The books are somewhat contentious in terms of quality, with complaints for the AD&D1e version especially about the overt focus on low/historical fantasy settings and the overabundance of clunky, unneccessary "like X class, but Japanese!" classes and oriental weapons.
Common Aspects of an Oriental Adventures setting:
Oriental Races for D&D include:
The 3rd edition version added Ratfolk (based on the Nezumi of Legend of the Five Rings) and Vanara. The Rokugan D20 Campaign Setting corebook features playable Naga and Nezumi. Pathfinder dumps the hengeyokai and the "Japanese Elves & Dwarves" of D&D for Kitsunes, Tengu, Vanara, Vishkanya and Nagaji, so far.
Campaign Settings associated with Oriental Adventures:
- Kara-tur - the OA region of the Forgotten Realms and basically the "original OA setting", losing out that role to Rokugan in 3e.
- Golarion - not fully, but the region of Tian Xia is basically the Golarion version of Kara-tur.
Though not an official D&D OA setting, the Magic: The Gathering plane of Kamigawa has a similar motif, and a Plane Shift on Kamigawa is perhaps the best chance of seeing an OA equivalent released for Dungeons & Dragons 5th Edition.