What was found
(CAN) THE NR 1 ENGINE HAD TO BE SHUTDOWN IN FLIGHT DUE TO A LOW OIL PRESSURE WARNING SHORTLY AFTER TAKE-OFF. THE AIRCRAFT RETURNED TO THE DEPARTED AIRPORT AND PERFORMED AN UNEVENTFUL SINGLE ENGINE LANDING. THE TROUBLESHOOTING REVEALED THE OIL CAP WAS SECURE AND THE OIL FILTER HOUSING O-RING WAS SPLIT. THE MAIN OIL FILTER AND IMPENDING BYPASS SWITCH WAS CHANGED THE NIGHT BEFORE DUE TO FAULT CODES. THE OIL FILTER WAS CHANGED DURING THE MORNING. THE O-RING AND THE OIL FILTER WILL BE REPLACED, THE OIL SYSTEM WILL BE FLUSHED BEFORE TO RETURN THIS ENGINE BACK TO SERVICE.