Inspired by this question: Can a druid get out of wild shape to avoid an opportunity attack?
In the cited question, the Druid wild shapes from a large creature back into a medium creature, placing themselves more than 5' from their opponent, thus provoking an OA.
By the OA rules,
The attack interrupts the provoking creature's movement, occurring right before the creature leaves your reach.
The rules say this interrupts the movement, but it's not clear from the Wild Shape rules when the Druid is relying on the beast's hit points and their own hit point pool
If the OA connects, which pool of hit points is the damage taken from?