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?
Best Answer
An Attack of Opportunity occurs when a target moves out of your reach (unless otherwise specified by a feat).
PHB pg. 195, Opportunity Attacks
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 Attack of Opportunity, it would say so.