This question already has an answer here:
Let's assume we are talking about a character with both the Polearm Master and Sentinel feats wielding a reach weapon. If an enemy enters my reach, and I hit him using the Polearm master's second perk, would the Sentinel Feat's first perk make him stop 10' from me or would he stop at 5' from me?
I think it would, but it also does seem a little too good to be true: the ability to force an opponent to wait for a full turn before making an attack, unless he happens to have a reach weapon?
To highlight how overpowered it could be: Fighter (Battlemaster) with the two feats in question and the Precision Attack maneuver wielding a halberd. Any single Opponent who uses melee attacks and doesn't have a long reach. Opponent's turn: Tries to come in range to hit me. OA from Polearm Master locks him down. If I miss, use Precision Attack to make it a hit. He can't use any attacks. Other turns: I (and any other melee teammates) all get to use all of our attacks, and then all step back to just 15 feet from him. The first person in initiative gets swung at by the enemy's OA, but everyone else is clear. In single enemy encounters this allows an easy pass for quite a few of them, with little regard for their stats. In duel situations, it golden.
Feats for reference. Irrelevant parts not included.
- While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter your reach.
- When you hit a creature with an opportunity attack, the creature's speed become 0 for the rest of the turn.