An O ring with a ragged edge or a strip missing from one side may have been forced into a clearance gap. That is extrusion. Repeated pressure pulses can trap and cut the material at the edge. The damage is normally on the low pressure side, downstream of the applied pressure. A fresh ring in the same gap may fail in the same way.
Keep the ring and mark the pressure direction and the damaged side. Photograph the damage before handling it further. Measure the groove and the clearance when the equipment is safe to inspect. Record normal pressure, pressure peaks and whether the gap changes as the equipment moves or heats up.
Check the material as well as the metalwork. A ring that has swollen or softened in the fluid may extrude more easily. A ring that is too soft for the clearance can also be vulnerable. A sharp groove edge or an overfilled groove can damage a correctly specified material.
The answer might be a smaller clearance, a different groove or a suitable support ring. It might be a change in hardness or material. Do not pick one from the torn edge alone. A clean cut made during installation can look like extrusion, so check how the ring was fitted as well. We need the pressure, fluid, temperature, groove dimensions and the location of the damage before we can separate those causes.
Related: Large Diameter O-Rings and Why is my O ring flattened and leaking after heat or cleaning cycles?.