The pilot reported that during cruise flight he observed an oil pressure warning indication on the electronic flight instrument system (EFIS) and noted that the engine oil pressure had dropped to approximately one pound per square inch (psi). He immediately turned toward the nearest airport, which was about 7 miles away. However, the engine stopped producing power about one minute later. The pilot prepared for a forced landing to a farm field. The field consisted of chest-high grass with soft, muddy areas. The airplane subsequently nosed over during the landing. A post-accident examination of the airplane revealed that an oil line between the oil cooler and the engine became disconnected. The oil line and fitting appeared to be otherwise intact. The examination also noted oil residue along the bottom of the fuselage. The oil line connection was not equipped with a clamp, and relied on the interference of the line and the fitting to maintain the connection. The pilot stated that the oil line manufacturer assured him that the design configuration was adequate.