A player is Out-Of-Position when he plays in a different position in real life than the one he was designated in the fantasy game.
OOP is applicable in fantasy sports where scoring depends on the player’s position and where positions can be fluid.
The most obvious example is football. Before the season starts, all player receive a position in the fantasy game. That position won’t change during the season. But it’s perfectly possible that his or her position changes in real life (due to a managerial decision, replacing an injured team mate).
A defender who plays in midfield, or a midfielder who plays as a forward, are the two Out-Of-Positions you need to look out for and possibly take advantage of. And the reverse is also true. Try to avoid having players in your team who play further back than they are designated in the fantasy game.