Can you Ready an action with someone's reaction as a trigger?

Question

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 Magic MissileFire 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

Answer 1

You can do this.

Your cat getting slapped is obviously a perceivable circumstance. Nothing wrong here.

"Perceivable" just means a circumstance you can perceive. You name your trigger. If you perceive it happen, you can use your held action.

Be aware, enemies may not be interested in slapping cats.

I wouldn't count on this working too often. Imagine a tense, heated battle between the party and some well armed bandits. You have this idea, so you send in your cat to harrass the bandit leader. Your cat runs around a bit then runs away. If I'm DMing, the bandit leader isn't wasting her reaction slapping your cat, and you never use your held action.

Remember: opportunity attacks are not mandatory.