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Enemies Abound
... On a failure, the target regards all creatures it can see as enemies, allies included. Each time the target takes damage, ...
... If an enemy provokes an opportunity attack, the creature must make that attack if it's able to.
Is "an enemy" the original enemy of the affected creature, or all creatures it can see ("modified enemy")?
Any creature that qualifies as enemy will qualify to trigger the opportunity attack. That would include both sets of creatures with that description. There isn't an exception, nor a distinction, made in the text for that spell so a plain English reading is the best way to avoid confusion.
Example:
Three creatures attack a monster who has two allies. At this point, the monster has three enemies: those three creatures.
The monster fails the saving throw after one of those creatures casts that spell, and it can see both of its allies who it now regards as enemies. That's a magical effect created by the spell.
At this point the monster has five enemies.
If one of its allies were originally invisible ...
... the monster would now have four enemies, since "that it can see" is a criterion for classifying another creature as an enemy and it cannot see its invisible ally.
Any of those creatures, should all other conditions for opportunity attack arise, are eligible to receive the opportunity attack as that monster's reaction.
