Does Aoo Occur When Spells Are Cast?

Cast a spell can sometimes trigger an Attack of Opportunity (AoO), depending on the components needed to cast the spell. Spells do not provoke or suffer any other penalties for being cast in melee, except those that require a ranged attack, which suffer disadvantage. 5ft steps do not trigger AoOs, but if the wizard moved, tripping them is an option. Casting spells with somatic components counts as a manipulate action, and an AoO can disrupt the spell (vampire touch). Reactions happen always before the current actions.

War Caster allows you to cast any spell with a casting time of one action instead of a melee attack when provoked by an opportunity attack. For most spellcasters, this is the case. When casting a spell, using a Spell-Like Ability or reading a scroll within a threatened area, you automatically attempt to cast defensively to avoid an AoO. Warcaster lets you cast a spell in place of an attack of opportunity and is therefore a replacement ability.

Casting a spell does not inherently trigger Attacks of Opportunity. Casting a spell with a casting time of 1 standard action or greater provokes an AoO, always. However, a caster can choose to try to cast a spell at the time they would normally make an opportunity attack, but that spell is not itself an opportunity attack.

An attack of opportunity (AoO) is a single (per round) free attack that is automatically taken when an enemy makes themselves vulnerable. Touch attacks come in two types: melee touch attacks and ranged touch attacks. Readied actions an AoOs happen before the action that triggered them. If you cast a spell, and someone readied to grapple you when you try to cast it, you should be able to cast Hellish.