The Tunnel Fighter fighting style from Unearthed Arcana: Light, Dark, Underdark! allows you to make Opportunity Attacks without using a reaction.
Tunnel Fighter: You excel at defending narrow passages, doorways, and other tight spaces. As a bonus action, you can enter a defensive stance that lasts until the start of your next turn. While in your defensive stance, you can make opportunity attacks without using your reaction, and you can use your reaction to make a melee attack against a creature that moves more than 5ft while within your reach.
Since you are no longer using a reaction, does this mean that when in this stance Opportunity Attacks no longer take an action?
Opportunity Attacks: ... You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. ...
This becomes important because there are spells that take an action to cast. eg
Casting Time: 1 action As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range ...
A tunnel fighter in their stance certainly meets the 'you must make a melee attack with a weapon' criteria. My question is does it still meet the 'Casting Time: 1 action' criteria.
The Specific beats general rubrik makes me think that the Opportunity Attack is now no longer an action of any sort. But that seems weird.
Somewhat related questions:
Tunnel Fighter & Polearm Master : Indefinite Attacks of Opportunity?
How does Tunnel Fighter interact with Paladin's Relentless Avenger?