D&D 5E Flanking and attack of opportunity.

[maritimo80]

If an enemy is flanked by two characters, and he in his action using a dash action, opportunities for attacks against him are with advantage? Or flank only advantage in attack and not in attack of opportunity?

 

[TDarien]

Flanking doesn't normally give advantage on an attack, Opportunity attack or not. If you're using the flaking option in the DMG... they still wouldn't get advantage. Since you only provoke an Opportunity Attack when you leave an enemy's reach, those enemies wouldn't be flanking you anymore when you provoked their attack.

Fig. 1
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-H--
-E--

Fig 2
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--H-
-E--

Fig 3
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---H
-E--

In the above example the Hero doesn't provoke an opportunity attack until he moves from position 2 to position 3. The enemies are not flanking the Hero in that position, so they do not get advantage on an Opportunity Attack.

 

[Dausuul]

If an enemy is flanked by two characters, and he in his action using a dash action, opportunities for attacks against him are with advantage? Or flank only advantage in attack and not in attack of opportunity?

The default 5E rules do not have flanking, so I assume you're using the DMG flanking option. If so, flanking will give advantage on all attacks. That includes opportunity attacks, if the creature provokes one while flanked.

However, as TDarien points out, it's very unusual for that to happen. In most cases, you'll move out of flanking before you leave the flankers' reach. To provoke while still flanked, you'd have to do something like this:

Fig. 1
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-H--
-E--

Fig 2
-EH-
----
-E--

In this case, the hero would provoke an opportunity attack from the enemy at the bottom, and that OA would have advantage from flanking.