From page 74 of the Basic Rules (also PHB p.195, or this SRD site):
[...] You can avoid provoking an opportunity attack by taking the Disengage action. You also don’t provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction. For example, you don’t provoke an opportunity attack if an explosion hurls you out of a foe’s reach or if gravity causes you to fall past an enemy.
If a creature is moved out of the threatened area (passive), rather than actively move (using its own action, movement or reaction), it does not provoke an Opportunity Attack.