Warcaster! Could a Cleric, with the War Caster feat, cast Cure Wounds as a reaction on an ally who provokes a “spell of opportunity” by leaving their melee range without disengaging. PHB pg 195: "You can make an opportunity attack when a HOSTILE creature that you can see…"

Emphasis mine. 🙂

— Dan Dillon 👥 (@Dan_Dillon_1) January 29, 2020