As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ORD INFLIGHT - LEFT ENGINE LOW OIL PRESSURE LIGHT ILLUMINATED FOLLOWED BY OIL QUANTITY DEGRADING. CREW SHUTDOWN ENGINE. NO HIGH OIL TEMPERATURES OR ENGINE GAS TEMPERATURES OBSERVED. RETURNED TO ORD AND LANDED WITHOUT INCIDENT. SINGLE ENGINE TIME APPROXIMATELY 20 MINUTES. REPLACED NUMBER ONE ENGINE ON 11 MARCH 2007 AT ORD DUE TO OIL PRESSURE OUT OF LIMITS. SYSTEM GROUND CHECKED NORMAL OPERATION.