This is a follow up to the question: When are Opportunity Attacks provoked while holding a reach and a non-reach weapon?
The answers establish pretty solidly that if I hold a weapon with a reach of 5 feet (say, a sword) and another weapon with a reach of 10 feet (say, a whip), an adjacent foe still triggers an Opportunity Attack if they move 5 feet away from me.
However, it is not clear to me if I can use either weapon for my Opportunity Attack, or if I have to use the weapon the enemy moved out of the reach of.
Can I use either weapon for the Opportunity Attack or does it have to be the weapon that the enemy leaves the reach of?
Best Answer
No, you're meant to make the opportunity attack with the weapon whose reach the enemy is leaving
The rules on opportunity attacks state:
Most melee weapons have a reach of 5 feet, as described in the rules for melee attacks:
To see how opportunity attacks work for weapons with a reach of 10 feet, we need to look at the description of the "reach" weapon property:
This specification about opportunity attacks - after the comma - was added in the first PHB errata in 2015.
When wielding a weapon with the reach property, enemies don't provoke an opportunity attack from it when the enemy moves from 5 feet away to further away. They only provoke an opportunity attack from it when they move from 10 feet away to further away.
The description of the reach property says it adds 5 feet to your reach "when you attack with it", including opportunity attacks "with it". The fact that it specifies that the extra reach applies to attacks with the weapon with that property makes it clear that you can't use your whip to attack a creature that provokes an opportunity attack based on your longsword's (5-foot) reach.
The Sage Advice Compendium officially answers a similar question:
As you can see, the reach of a weapon is what determines when an opportunity attack is triggered for that weapon.
(Technically, you could always make an opportunity attack, regardless of wielded weapons, when a creature that's next to you moves further away - using an unarmed strike. But for most characters, that's not worth it.)
This is further supported by several unofficial tweets by 5e rules designer Jeremy Crawford.
He originally mentioned it in passing in 2014 in response to a question about the Polearm Master feat. The other user seems to have deleted their account, but SageAdvice.eu retains the text of the tweets so I've reproduced them below:
(At the time, the only weapon without the reach property that the Polearm Master feat (PHB, p. 168) benefited was the quarterstaff; the 2018 PHB errata added spears to the weapons that benefit from it as well.)
Crawford reiterated this in another series of tweets in 2016; the initial one asked about monsters' opportunity attacks, but the followups apply the same logic to PCs:
Here, Crawford clearly points out what the phrase "your reach" as used in the rules means. Reach isn't an inherent aspect of creatures, but rather of weapon attacks. By default, it's 5 feet; the "reach" property changes it to 10 feet for the specific weapons that have it.