A MOONEY M20C COLLIDED WITH AN AUTOMOBILE DURING LANDING ROLL AFTER LANDING ON A ROAD. THE PLT INDICATED THAT WHILE CRUISING AT 3,500 FT MSL HE NOTICED THE ENG OIL PRESSURE FALL TO ZERO. THE CONSTANT SPEED PROPELLOR SIMULTANEOUSLY WENT TO LOW PITCH, FOLLOWED BY THE ENG SEIZING. THE PLT WAS FORCED TO LAND AND SELECTED A ROAD TO LAND ON. ON FINAL APPROACH THE PLT OBSERVED TWO AUTOMOBILES HAD DRIVEN INTO HIS PLANNED LANDING AREA. DURING THE LANDING ROLL THE PLT WAS ABLE TO AVOID THE FIRST AUTOMOBILE, BUT COLLIDED WITH THE SECOND. THE ACFT LEFT WING WAS SUBSTANTIALLY DAMAGED IN THE COLLISION. THE AUTOMOBILE WAS ALSO DAMAGED AND ITS DRIVER SUSTAINED MINOR INJURIES. EXAMINATION OF THE ACFT ENG REVEALED THE ENG OIL DRAIN PLUG WAS MISSING WITH NO EVIDENCE OF IT BEING SAFETY WIRED. THE ENG HAD COMPLETED AN ANNUAL INSPECTION 4 DAYS PRIOR TO THE ACCIDENT AND HAD ACCUMULATED 2.9 HOURS OF OPERATION SINCE A MAJOR OVERHAUL. THIS WAS THE ACFTS SECOND FLIGHT SINCE THE ENG WAS OVERHAULED.