WHILE IN CRUISE FLIGHT, THE PILOT NOTED AN UNSUAL SMELL WHICH HE THOUGHT MIGHT HAVE BEEN ELECTRICAL OR SMOKE. SHORTLY THEREAFTER, THE AIRPLANE'S ENGINE OIL PRESSURE DECREASED. THE POWERPLANT THEN STARTED TO VIBRATE AND THE PILOT SHUT IT OFF. THE PILOT WAS COMMUNICATING WITH AIR TRAFFIC CONTROL AND WAS ATTEMPTING A PRECAUTIONARY LANDING AT AN AIRPORT WHEN THE FORCED LANDING OCCURRED. THE AIRPLANE WAS LANDED IN AN OPEN FIELD APPROXIMATELY ONE MILE FROM THE END OF THE RUNWAY THAT THE PILOT HAD BEEN CLEARED TO LAND ON. THE AIRPLANE FLIPPED OVER DURING THE LANDING ROLL ON THE UNSUITABLE TERRAIN. POST ACCIDENT EXAMINATION REVEALED THAT AN 'O' RING IN THE OIL FILTER ADAPTOR ASSEMBLY WAS CRUSHED AND THE ENGINE OIL ESCAPED AT THAT DAMAGED AREA CAUSING THE LOSS OF POWER. NO OIL REMAINED IN THE ENGINE. THE ADAPTOR ASSEMBLY HAD BEEN WORKED ON THE DAY BEFORE THE FLIGHT BY OTHER MAINTENANCE PERSONNEL.