The rule for readied action says:
First, you decide what perceivable circumstance will trigger your reaction.
Of course, it's rather hard to be ready for someone's else reactions, especially first-met enemies. I'm also aware that what got me thinking about this (the trigger being someone's attack of opportunity) is very much a meta-thinking (I, as a player, can be well aware of what actions will almost always trigger AoO's. Characters in game - not so much). However, I didn't see anybody asking this question, so what do you think?
Here's a specific example: a wizard readies a
Fire Bolt with a trigger "when my familiar gets attacked" and then on the familiar's turn it sends it within 5ft. of an enemy to do a Help action to aid himself in attacking, and then moves it out of the enemy's melee reach, which will in turn trigger an AoO, hence a triggering attack.
Also, I saw this answer, but it doesn't answer my question.
I posted a follow-up question here