Some concentration spells give you a special action. Does the War Caster feat (PHB, p. 170) let these be used instead of opportunity attacks?
For a specific example, the dragon's breath (XGtE, p. 154) and sunbeam spells both have actions to do damage after being cast. Could that action be used in that circumstance?
War Caster states:
When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, instead of making an opportunity attack.
When you use the action of such spells to deal damage, you are not casting the spell again1. Therefore, the War Caster text does not apply.
Note that this is, by itself, enough to preclude the use of such actions in the reaction replacing the opportunity attack. War Caster provides a specific benefit with specific wordings which are not the same used in the actions (i.e., the action is not casting a spell).
1 Does Dragon's Breath break Sanctuary? - as the premise states: "We know that a familiar can use Dragon's Breath establishing it is not an attack, nor is it a spell being cast."
This is further clarified in this answer to the question "Can a spell be cast while using a bonus action to control another spell?".
In some specific cases, there is yet another reason: Dragon Breath's action, for example, states
Until the spell ends, the creature can use an action to exhale energy of the chosen type in a 15-foot cone.
while the War Caster requires
The spell must have a casting time of only 1 action and must target only that creature.
While this does not provide a text as clear and unambiguous as, for example, Twinned Spell (in which the spell can not be able to target more than one target), this is another possible reasoning for a DM to not allow such action. However, targeting in 5e raises quite a few doubts, problems and discussions (check, e.g., our targeting tag section), thus I'm just mentioning it as a possibility for DM ruling.
