Following a normal touchdown and landing roll, the pilot commenced a left turn to backtrack along the runway to the parking area. During the turn the right main landing gear leg collapsed. An inspection of the gear leg revealed an incorrectly seated O-ring, which had allowed hydraulic pressure to bypass a section of the oleo. This resulted in the internal locking pins becoming unlatched prematurely when the assembly was subjected to side loads during the turn at the end of the runway. Records showed that the O-ring had recently been replaced, and it is likely that the distortion of the ring occurred during the oleo re-assembly process.