Shortly after departure, when the airplane was approximately 250-feet AGL (above ground level), oil began to “spray” on to the windshield. The pilot then checked the engine gauges and noticed a drop in oil pressure. As the pilot turned to the crosswind leg, the engine started to run “rough”, and he elected to conduct a 180-degree turn back to the runway. The airplane landed on about the last 250-feet of the runway and proceeded through a fence. The airplane sustained structural damage during the impact with the fence and came to rest upright, among several parked buses. An examination of the airplane by a Federal Aviation Administration Inspector, revealed that the engine’s oil cap was off and hanging by its chain. The cap’s gasket appeared “worn”; however, the inspector could not determine when the cap came off. He added, that the pilot stated that the engine was serviced with oil prior to the flight, but the oil cap was secured.