Can you take an Attack of Opportunity on a party member?

@JeremyECrawford Can you take an AoO on a party member? Say he was fleeing because he was frightened, could you trip him?

— Jonny Ward (@darth_dom) April 27, 2016

An opportunity attack is triggered by a foe. (The game won't break if a DM lets it work against an ally.) #DnD https://t.co/0eIk5i3JaE

— Jeremy Crawford (@JeremyECrawford) May 3, 2016

Without a special ability, you can't trip someone with an opportunity attack, which is a plain melee attack. #DnD https://t.co/0eIk5i3JaE

— Jeremy Crawford (@JeremyECrawford) May 3, 2016