Does a Monster with a body part that grants Reach provoke an Opportunity Attack?

Question

Consider a wyvern, that can reach out with its long neck and bite an enemy, or an otyugh which can reach out with a long tentacle for a smack and grapple.

By a strict RAW reading, it would seem that neither of these monsters would provoke an opportunity attack since the base creature stays out of reach of most characters. Yet within the reality imagined by the game, some part of their body is within the reach of character during that attack, and that part leaves the reach of that character during the attack. It seems like that should qualify for triggering an OA.

Maybe this is addressed in the Monster Manual, but I don't have my books with me at the moment. Is this seeming conundrum addressed anywhere, or does a body part follow the same rules as reach with a melee weapon and not trigger an OA?

Answer 1

An Opportunity Attack occurs when a target moves out of your reach (unless otherwise specified by a feat).

PHB pg. 195, Opportunity Attacks

You can make an opportunity attack when a hostile creature that you can see moves out of your reach.

The key word there is moves. Unless the monster's attack involves it actually moving out of your reach, it does not physically do so, and does not trigger an opportunity attack. The reach characteristic does not state that the creature making the attack is within striking range, regardless of whether or not a part of it is at any time for the duration of the attack.

Mechanically, the game sees no difference between a Polearm attack, an Otyugh slap, or a Wyvern bite. If the ability provoked an Opportunity Attack it would say so.

Answer 2

An opportunity attack is when a creature drops its guard

PHB 195:

In a fight, everyone is constantly watching for enemies to drop their guard. You can rarely move heedlessly past your foes without putting yourself in danger; doing so provokes an opportunity attack.

This passage places its emphasis on not being careful during movement. However, an attack is the opposite of that. There is a significant difference between trying to hit a creature's snapping mouth as it retracts and trying to hit the backside of a creature as it runs away.

Moreover, your argument would apply to non-reach attacks too. What about a bear's bite? In the fictional reality, it's the same as a wyvern's bite, in that it reaches part of its body out and pulls it back. Is the difference only the length?

Creature size accounts for these large body parts

PHB 191:

A creature's space is the area in feet that it effectively controls in combat, not an expression of its physical dimensions.

A wyvern or an otyugh is large, even though it might not take up the full 10x10ft. Being large means that exposes itself to more opportunity attacks, because more creatures can surround it. In this case, when it moves out of the reach of an enemy, one could imagine that it's main body is already pretty far away, and it's only the long tentacles or the long neck that's exposed. Because it's no longer actively attacking, it becomes vulnerable to an opportunity attack.