What actions can monsters use to make Attack of Opportunity?

@JeremyECrawford What actions can monsters use to make Attack of Opportunity? Are Multiattack and Breath Weapon actions allowed?

— Leonardo Raele (@LeonardoRaele) January 22, 2016

A monster follows the normal opportunity attack rule (PH, 195). It's one melee attack. #DnD https://t.co/6bSSl6w4am

— Jeremy Crawford (@JeremyECrawford) January 23, 2016