The pilot reported that the airplane was operating normally during the engine checks prior to takeoff. About 10 minutes after takeoff, he was climbing to 2,500 feet above mean sea level when he noticed that the airplane was not gaining altitude and that the rpms were dropping. He checked the throttle position at full forward and he pulled the carburetor heat on. He then noticed that the oil pressure gauge was reading zero. The airplane continued to lose altitude, with the propeller windmilling and the rpm gauge reading about 1,000 rpm. He executed a forced landing to a wooded area since there were residential homes in the area and no open fields available. Postaccident inspection of the airplane revealed no engine oil leaks and its oil level was at capacity. There was 2.5 gallons of fuel in the right wing tank and 1.3 gallons in the left fuel tank. The system capacity is 12 gallons total with 12 gallons of usable fuel. No water or contaminants were present. With the wings removed, the engine was started and 30+ psi of oil pressure was indicated on the oil pressure gauge within five seconds at idle power. The inspection revealed no deficiencies that would have precluded normal engine operation.