What was found
OKC - ON DEPARTURE, FLIGHT CREW NOTICED THE NR 1 ENGINE OIL QUANTITY WAS DECREASING. THE ENGINE WAS PRECAUTIONARILY SHUT DOWN, AND THE AIRCRAFT RETURNED TO OKC FOR MAINTENANCE. SOME OIL QUANTITY WAS RETAINED, AND MINIMUM WINDMILLING OIL PRESSURE WAS MAINTAINED. ON THE GROUND IN OKC, DETERMINED THE NR 1 ENGINE OIL TANK CAP WAS NOT LOCKED ALLOWING LEAKAGE AROUND THE CAP. THE ENGINE WAS SERVICED, THE OIL CAP SECURED, AND THE AIRCRAFT RETURNED TO SERVICE. (X)