OTG stands for the phrase "On The Ground". It is used on one of several ways in Super Smash Bros. Battle Royale, as well as other fighters.
An attack that hits OTG is simple: This means the attack will hit an opponent who is laying down on the ground. This really only applies to Final Smashes, as most moves will hit an opponent who is lying on the ground. This is a Smash Bros. game.