The French club paid a club-record €30m to Gent for the 20-year-old Canadian striker.
"I'm really happy. I've been eagerly awaiting this day and it's a pleasure to be a Dogue," David said. "I know the project that has been put in place here by the president, Luis Campos, and [head coach] Christophe Galtier.
"This is a very good team with great players. I want to play and to progress and I think that this is the perfect place. I'm a player who knows his place on the pitch. I love to play to my qualities, notably with speed and and technique. I'm also eager to see our fans! Let's go LOSC!"