OOP is a simply an acronym that stands for out of position . Being out of position  means to act first on each betting round. Being out of position is considered to be a disadvantage since the out of position player does not get to see what his opponent does before acting on each street.

The spots or marks on the face of a card. When used as a verb, the term refers to a card beating another by a single pip   i.e., outranking another by one. 

Pluribus's self-play produces a strategy for the entire game offline, which we refer to as the blueprint strategy. Then during actual play against opponents, Pluribus improves upon the blueprint strategy by searching for a better strategy in real time for the situations in which it finds itself during the game.

Secondly, the broad set of hidden information of the tiles makes Mahjong a much more difficult imperfect-information game than previously studied ones, such as Texas hold'em poker. Thirdly, the playing rule of Mahjong is much more complicated because of the various actions involved.