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?