Can some monsters grapple as an opportunity attack?

Question

In 5e, I realize that you can not grapple as an opportunity attack, as the grapple is an action and the opportunity attack is a reaction.

However, am I correct in assuming that some monsters can grapple as an OA?

Opportunity Attack

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature.

Let's use a Vampire Spawn as an example. Listed in its attacks is using its claws:

Claws. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 8 (2d4 + 3) slashing damage. Instead of dealing damage, the vampire can grapple the target (escape DC 13).

Since the claws are considered a melee attack and they have this extra ability to grapple instead of deal damage, I would think they could be used for an AoO.

Just wanted to see if anyone can point out something I may have missed. Thanks!

Answer 1

Yes, as long as the grapple is delivered by a melee attack that doesn't require some other specific Action

The sage advice compendium answered a similar question to this one about spell attacks. In part, it talks about monsters making spell attacks as opportunity attacks. The clarification notes that:

A few monsters can make opportunity attacks with melee spell attacks. Here’s how: certain monsters—including the banshee, the lich, and the specter—have a melee spell attack that isn’t delivered by a spell. For example, the banshee’s Corrupting Touch action is a melee spell attack but no spell is cast to make it. The banshee can, therefore, make opportunity attacks with Corrupting Touch.

From this, we can determine that a monster can use any of its melee attack options when it makes an opportunity attack, as long as they don't require some other action to use (like how the grapple or shove attacks in the PHB require the Attack action).

So, the vampire spawn can use its claw attack as an opportunity attack since it's a melee attack that is not mandated to be delivered by some other specific action. The claw attack itself can grapple a target if it hits instead of dealing damage. And since opportunity attacks do not specifically override any on-hit effects from making an attack, the vampire spawn can elect to replace the damage roll with the grapple.

Answer 2

No to the Vampire, because Claws is also a Special Action

Just like how making a Special Attack like Grappling or Shoving is a special action and not an Attack of Opportunity, the Vampire Spawn's stat-block reads:

Actions

...

...

Claws: Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 8 (2d4 + 3) slashing damage. Instead of dealing damage, the Vampire can grapple the target (escape DC 13).

Thus, a Vampire can only use Claws as an Action and not a Reaction.

Some monsters can grapple as an Attack of Opportunity. However, these will typically say so explicitly.