My DM has introduced a house rule affecting opportunity attacks, and I am trying to understand the implications of this change.
The new rule is this:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach or moves inside your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack interrupts the provoking creature’s movement, occurring right before the creature leaves your reach.
I've bolded the portion added by the house-rule.
To clarify, when a creature moves from out of a character's reach into their reach, it does not provoke an opportunity attack. But if a creature moves within a character's reach (i.e. from one spot within reach to another), it provokes an opportunity attack.
I think this can reduce the utility of the Rogue (with Sneak Attack), and kind of reduce the Mobile feat's benefits. Are there other places where this rule can change how combat is played? Like, what can become imbalanced with this ruling?
My goal is to better understand this mechanic before talking with my DM.
Best Answer
This is going to make your combats a lot more static. Nobody likes taking opportunity attacks, and it costs an action to get away without one (rogue aside), so everyone who likes melee will tend to get into position and then stand very still -- even being in a poor tactical position would be better than taking OAs or giving up a turn to change it.
A big consequence of this is that any abilities that depend on precise positioning are much less useful. For example, the Protection fighting style depends on being adjacent to a friend, and this rule will make it more difficult to set that up, so you should probably go with a different style instead. Similarly, close range spells like thunderwave may be more difficult to use effectively, because your allies will have less ability to move away without taking OAs for doing it.
Conversely, effects that prevent reactions (such as the shocking grasp cantrip) become stronger, since they effectively remove the ability to make OAs and thus free up melee characters for movement. This includes conditions like incapacitated, stunned, or paralyzed.
It won't change things much for your range-focused combatants, who generally just want to get away if they find themselves in melee and will therefore take OAs anyway.
What you're describing is a throwback to 3rd edition, only without the five-foot step to help ameliorate the problems it causes. The 5-foot step was awkward and weird, but still better than effectively locking everyone in place once they engage in melee.