Assume I have a Familiar, per the spell Find Familiar, and we're each adjacent to different enemies. If an enemy provokes an opportunity attack from me, and I have War Caster and a spell that targets only one creature, has a range of touch, and has a casting time of 1 action, can my Familiar attack the enemy adjacent to it, which did not provoke the opportunity attack?
Per Find Familiar, page 240 PHB 5e,
...when you cast a spell with a range of touch, your familiar can deliver the spell as if it had cast the spell. Your familiar must be within 100 feet of you, and it must use its reaction to deliver the spell when you cast it.
War Caster, page 170 PHB 5e,
... When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.