The VOIDRUNNER'S CODEX is coming! Explore new worlds, fight oppressive empires, fend off fearsome aliens, and wield deadly psionics with this comprehensive boxed set expansion for 5E and A5E!
If a fighter Mark an opponent, he gets to attack him if that opponent shift away. But it's actually an immediate action, not an opportunity attack (You only get one immediate action/turn).
A warlord using Viper strike could grant an ally an AoO against his taget if he shifts away.