Hydraulic couplers with O-ring seals provide various advantages over metal to metal connections. Metal hydraulic fittings are more prone to over-tightening which can distort fitting components and allow leakage. Excessive torque can allow leakage by stripping threads or cracking components. A hydraulic coupler with an O-ring fitting provides a metal to rubber seal which does not distort metal components. Additionally, the metal to rubber seal provides a tangible feeling when connection is too tight and is easier to detect when a fitting has been over tightened to prevent future problems.