With the Tunnel Master fighting style (from Unearthed Arcana: Light, Dark, Underdark!), a character can use a bonus action to take a defensive stance that lets them make opportunity attacks without expending a reaction.
The Oath of Vengeance paladin gets the Relentless Avenger feature at 7th level (PHB, p. 88); when he makes an opportunity attack, he can move up to half his speed as a part of the reaction used to make the attack.
Can a character in a Defensive Stance with Relentless Avenger move with every opportunity attack? Can he move only once, expending his reaction, while still having unlimited opportunity attacks otherwise?