The pilot reported that during the local flight the engine experienced a loss of oil pressure and the cockpit began to fill with smoke. He immediately reduced engine power to idle and performed a forced landing to a nearby field. The airplane collided with a barbed-wire fence during the landing rollout and subsequently nosed-over. The fuselage, right wing, and vertical stabilizer were substantially damaged. A postaccident inspection revealed that the oil supply line to the oil cooler had ruptured, which allowed oil to come in contact with the engine's hot exhaust producing smoke. The rubber, automobile-style, oil line had failed immediately adjacent to its respective oil cooler inlet fitting. The hose-clamp used to secure the oil line to the inlet fitting was not properly positioned over the fitting, which allowed the unsupported hose-clamp to abrade through the oil line.