How does the Overrun action work?

Question

The action 'Overrun' is described in the DMG p. 272 as:

When a creature tries to move through a hostile creature's space, the mover can try to force its way through by overrunning the hostile creature. As an action or a bonus action, the mover makes a Strength (Athletics) check contested by the hostile creature's Strength (Athletics) check. The creature attempting the overrun has advantage on this check if it is larger than the hostile creature, or disadvantage if it is smaller. If the mover wins the contest, it can move through the hostile creature's space once this turn.

This action is an optional addition to the game.

It doesn't explain what condition the overrun enemy is left in, or clarify if it could make an attack of opportunity. How do you rule it when DMing?

Answer 1

Overrun is a bonus-/action that lets you move through a hostile creature's space.

Overrun lets you move through a hostile creature's space of any size and gives you mechanics to do so (contesting check as a bonus-/action). It adjusts the rules for Moving Around Other Creatures (PHB 191):

You can move through a nonhostile creature’s space. In contrast, you can move through a hostile creature's space only if the creature is at least two sizes larger or smaller than you. Remember that another creature’s space is difficult terrain for you. Whether a creature is a friend or an enemy, you can’t willingly end your move in its space. If you leave a hostile creature’s reach during your move, you provoke an opportunity attack[.]

Overrun does not change the condition of any creature involved. The regular rules of opportunity attacks apply: if you leave the reach, then you provoke an opportunity attack (for more information about opportunity attacks, read PHB 195).

So if you move through its space, but do not leave the reach of the creature, then you do not provoke an opportunity attack by moving through its space with the Overrun bonus-/action.