A game, originating in San Diego, California. OTL stands for "Over the Line" Similar to softball where there is a batter, pitcher, and outfield.
The objective of the game is to get more homeruns than the opposing team. The batter tries to hit the ball (thrown upwards by the pitcher from the ground) with a bat and get it to fly over a line on the field. If it is caught midair, the batter is out. If it lands on the ground or is touched by one of the outfielders then hits the ground/goes out, then that counts as one base. If the ball flies past the farthest outfielder's head, it's a homerun.
PE Teacher: "Our next unit in PE this quarter is going to be OTL."