The third benefit of the War Caster feat (PHB, p. 170) allows you to cast a spell when an enemy provokes an opportunity attack from you. The spell must target only the provoking creature, and can only have a casting time of 1 action.
For the sake of abilities like the first benefit of Sentinel (which causes opportunity attacks to reduce a creature's speed to 0), does my spell cast using the third benefit of War Caster still count if it uses an attack?
Best Answer
No, it's not an opportunity attack
The relevant phrasing in the Sentinel feat (PHB, p. 169-170) is:
So the prerequisite for this feature is that you must hit a creature with an opportunity attack.
The relevant phrasing in the War Caster feat (PHB, p. 170) is:
While the trigger is a situation that provides an opportunity attack, it specifically states that no opportunity attack is made, and you instead us a reaction for the spell casting. Thus, the Sentinel feat is not triggered.