I believe that Quagsire is OU because of his capabilities of being a defensive wall. Quagsire's Hidden Ability is Unaware, where it makes it so that it ignores all positive and negative stat changes of the opposing Pokémon (excluding Speed). Quagsire also has access to the moves Stockpile and Recover, allowing it to set up both Defense and Special Defense boosts in front of people's faces and then regain most of its HP back with Recover. Quagsire can stall with Toxic, or try to fish for a burn when using Scald. Basically, Quagsire does what Swampert doesn't. Either way, I think Quagsire is a great Pokémon and deserves to be in OU. I hope this helped!