Opportunity Attack, Ready Action, & Eldritch Blast/Repelling Blast

Question

Scenario:

A druid ally is in melee range of an enemy berserker and decides to move away, provoking an attack of opportunity from the berserker.

My character has readied an action: Cast Eldritch Blast (with Repelling Blast invocation) on the berserker in question if he attacks the druid.

What I think happens

The druid's movement is interrupted by the attack of opportunity which is in turn interrupted by the readied Eldritch Blast. Assuming the Eldritch Blast hits, I can push the berserker out of melee range of the druid, thereby negating the attack of opportunity.

Is this correct?

Similar situations

What if, rather, the trigger was the druid moving? What if the trigger was the berserker raising his weapon or rearing back in preparation for an attack?

Answer 1

Your Druid Gets Attacked, then the barbarian gets pushed

The trigger for your readied action was:

Cast Eldritch Blast (with Repelling Blast invocation) on the berserker in question if he attacks the druid.

It is stated in the Dungeon Master's Guide (p. 252) that:

If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes, as in the Ready action.

So your readied Eldritch Blast will occur right after the berserker attacks the druid.

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What if, rather, the trigger was the druid moving? What if the trigger was the berserker raising his weapon or rearing back in preparation for an attack?

The alternative triggers would not improve matters much. If you specified the trigger was "the berserker raises his weapon to attack", that might be tricky, because the trigger has to be a "percievable circumstance." In combat, everyone always looks like they're just about to attack each other: what if the enemy doesn't "raise" a weapon, but stabs with it? What if they raise a weapon in salute?

If the tirgger was the Druid's movement, that's no good either because the Barbarian will attack before the Druid moves away from his reach (as that's the timing specified in Opportunity Attacks), and you'd Eldritch Blast after the Druid finished moving away from his reach (since it's a Readied action, and happens after its trigger), so the berserker would still attack first.

A relevant question that has been asked before is can I specify a trigger to be "before... occurs"? (Mostly, you can't).

Answer 2

You cannot prevent the barbarian from finishing its attack on the Druid

As per Ready action rules (emphasis mine)

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.

No matter how you state the trigger, the action trigger will always complete before the ready action is performed. Also note that the trigger must be a perceivable in-game action (a movement, any of the actions in the PHB or something similar)

So in your example you could have stated as a trigger

1. When the barbarian attacks the druid
2. when the druid moves
3. When the druid leaves the reach of the barbarian

None of the resulting ready action would allow you to cast the spell before the barbarian finishes the attack on the druid. I actually don't have anything coming to mind that you could have used as a trigger to prevent it.