As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLIGHT CREW REPORTED THAT THE LEFT ENGINE OIL PRESSURE DROPPED AND OIL CAPTION LIGHT ILLUMINATED AFTER TAKEOFF. THE ENGINE WAS SHUTDOWN AND RETURNED TO THE FIELD. MAINTENANCE INSPECTED THE AIRCRAFT AND FOUND THE O-RING ON THE OIL FLAPPER VALVE IN THE OIL SERVICING PORT BROKEN. THE O-RING WAS REPLACED, OIL SERVICED, GROUND RUNS AND LEAK CHECKS PERFORMED WITH ALL INDICATIONS NORMAL.