They interrupt attacks and are too frequent. Several times by now did an opportunity attack interrupt my own attack and made the character loose its action.
E.g.: It's Lae'zel's turn. A longsword is held by her. I click on an enemy to attack it. Lae'zel runs closer in a straight line and triggers an opportunity attack (Why?). The enemy uses its own longsword to swing at Lae'zel. Lae'zel can not make her own attack and it's counted as used up.
This looks more like a bug, than a deliberate gameplay choice.
This is definitely a bug that is known by Larian. I got a response from my bug report about it on October 15 saying that they're working on a fix for a future update.