How does this change to the opportunity attack rule impact combat?

Question

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.

Answer 1

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.

Answer 2

Consider the following Cavalier fighter archetype feature (XGtE, p. 30):

Hold the Line

At 10th level, you become a master of locking down your enemies. Creatures provoke an opportunity attack from you when they move 5 feet or more while within your reach, and if you hit a creature with an opportunity attack, the target’s speed is reduced to 0 until the end of the current turn.

Your DM's house rules seem to give a feature to everyone that is awarded to a level 10 fighter (without the reduction of speed to 0). This, along with Darth Pseudonym's answer above, will have a significant impact on combat.

If your DM allows UA content as well, the Tunnel Fighter fighting style is also very good.

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.

Answer 3

In addition to the tactics changes that Darth P. mentioned, it cheapens the Polearm Master feat and any other similar feats or abilities that grant opportunity attacks when an enemy approaches.

Answer 4

One consequence is, that it makes combat more believable, as seen in these situations:

• Someone charging a polearm-bearer with a dagger will take an opportunity attack.
• Depending on how you read your GM's rule, standing up now triggers opportunity attacks.

In 5e it can be quite frustrating that tripping an enemy does often not have the desired effect. Depending on the turn order, that creature might just spend half its movement standing up and continue attacking as if nothing had happened.

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If you are worried about mobility, consider asking your DM to introduce a 5-foot step (wording mine):

5-foot step: A creature may use their movement to only move 5 feet, in which case it is treated as if it had taken the Disengage action.

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If you are worried about polearm master being lackluster or polearms being overpowered due to your GMs houserule, you might want to propose this additional rule:

Any attacks made using a weapon with reach against an enemy within 5ft are made at disadvantage. Creatures with the Polearm Master feat ignore this.

This brings reach weapons in line with ranged weapons and ranged spells.