On August 31, 1993, at 1100 central daylight time, a Beech P35, N8527M, registered to Darrell W. Allen of Independence, Missouri, and operated by an instrument rated private pilot, experienced a loss of oil pressure while in cruise flight. During a forced landing in a corn field one and one-half miles north of the Ottumwa Industrial Airport, Ottumwa, Iowa, the airplane sustained substantial damage. The pilot and one passenger reported no injuries. The personal 14 CFR Part 91 flight was operating in visual meteorological conditions. No flight plan was on file. The flight departed Independence, Missouri, at 0955 with the intended destination of Amana, Iowa. The pilot stated that when oil pressure went to zero (0), he attempted to fly to the airport. When he realized that he would not be able to make the airport, he elected to initiate a forced landing in a crop field. An examination of the engine, by a Federal Aviation Administration inspector revealed an oil leak from a seal in the oil filter adapter.