Monster oppertunaty attacks, when not all attacks have same reach

[Alexander Atoz]

Some monsters have more than one type of attack, and the different attacks have different reaches. For example, adult and ancient dragons have 3 attacks, all with different reaches. Does this mean that an attack of opportunity is only triggered when the PC leaves the range of the attack with the furthest reach? Seems weird, but also seems like the most literal reading of the rules.

[InvisibleBison]

The rule for opportunity attacks says "You can make an opportunity attack when a hostile creature that you can see moves out of your reach." If you have different reaches with different attacks, I don't see why you wouldn't have the option to take an opportunity attack whenever an enemy moves out of reach of any of them.

[Amnestic]

RAW, if it leaves any of your reaches, you can make an OA.

Dual wielding a shortsword+whip, if a creature moved from adjacent to not-adjacent, you would still get an OA. The rules do not specify, strictly, which weapon(s) you have to use, therefore you could make an argument that you can attack with any weapon you have, even if it's one that didn't leave your reach on (whip, in this example) and it'd be up to DM decision if that's valid. Personally I'd say 'no'.

For what it's worth, Sage Advice Compendium (which are 'official rulings') specifies that it does have to be the relevant weapon:-

How does a reach weapon work with opportunity attacks?

An opportunity attack is normally triggered when a creature you can see moves beyond your reach. If you want to make an opportunity attack with a reach weapon, such as a glaive or a halberd, you can do so when a creature leaves the reach you have with that weapon. For example, if you’re wielding a halberd, a creature that is right next to you could move 5 feet away without triggering an opportunity attack. If that creature tries to move an additional 5 feet—beyond your 10-foot reach—the creature then triggers an opportunity attack.

As such, in the event of a dragon who has claws (5ft), bite (10ft), and tail (15ft), they have three ranges of opportunity attack to choose from, but each one would only be used for the specified range, rather than tailing someone going from 5ft->10ft. Assuming you stuck to SAC, anyway.

But they'd still provoke from claws going from 5ft->10ft, because they've left the Reach of that weapon.

[Alexander Atoz]

Thank you very much