Question about the Sentinel Feat???

We had a session yesterday. We fought a hag and her two daughters, and some of them made attacks with spells which required a saving throw instead of an attack roll. Based on the wording of Sentinel, it seems to me that I should still be able to make an opportunity attack with Sentinel even on spells that don't require an attack roll? What's everyone's opinion on this?  TIA

Sentinel

General Feat (Prerequisite: Level 4+, Strength or Dexterity 13+)

You gain the following benefits.

Ability Score Increase. Increase your Strength or Dexterity score by 1, to a maximum of 20.

Guardian. Immediately after a creature within 5 feet of you takes the Disengage action or hits a target other than you with an attack, you can make an Opportunity Attack against that creature.

Halt. When you hit a creature with an Opportunity Attack, the creature’s Speed becomes 0 for the rest of the current turn.

No, you cannot.  Guardian requires a target to be hit with an attack. Saying "some of them made attacks with spells which required a saving throw" is a misnomer; spells that require saving throws are not attacks. Only actions that result in making an attack roll vs a target's AC are attacks.

I don't understand how a spell that does damage to an ally is not considered an attack?

Makes sense, it appears that the matter is settled.

Thanks for the input!

In case it's helpful, this thread is related: Using Sentinel Feat against a Spell Caster - Rules & Game Mechanics 

Quote from TarodNet >>

In case it's helpful, this thread is related: Using Sentinel Feat against a Spell Caster - Rules & Game Mechanics 

 Would I be able to use Sentinel as an opportunity attack against a spell caster within 5' of me, if they make an attack roll with a spell that hits one of my allies? 

Quote from LeoDillon >>

Quote from TarodNet >>

In case it's helpful, this thread is related: Using Sentinel Feat against a Spell Caster - Rules & Game Mechanics 

 Would I be able to use Sentinel as an opportunity attack against a spell caster within 5' of me, if they make an attack roll with a spell that hits one of my allies? 

 Let me add here this answer from @Plaguescarred:

Quote from Plaguescarred >>

Tulbar Ok i see now what you meant, allow me another take;

A creature provoke an Opportunity Attack if it hits a target other than the Sentinel with any attacks, wether melee, ranged, with a weapon or spell.

If the spell or effect is a result from an area of effect or static damage, it will not provoke because it must hit and therefore make an attack roll.

The Sentinel War Caster only Halt when hitting with an Opportunity Attack, not when casting a spell at the creature rather than making one.

EDIT: ninja'd by wagnarokkr :D