The pilot reported that during the flight he noticed the oil pressure gauge fluctuating and executed a precautionary landing to a road to examine the engine. He stated that after landing, he performed a walk around inspection of the airplane, which included looking for leaks and checking the oil level and security of the oil cap. Finding no anomalies, he proceeded to take off from the road. During takeoff and while the airplane was gaining airspeed in ground effect, the departure path went over a terrain depression and into a down sloping bowl; the airplane encountered a sudden sink/down flow of air and lost about 10-15 ft of altitude. The pilot attempted to turn toward lower terrain while maintaining airspeed but the sink/down flow of air continued and so he maneuvered to land the airplane in a meadow. Unable to reach the meadow, he turned the airplane into sloping terrain to minimize the impact energy. During the impact, the airplane sustained damage to both wings, the fuselage, and empennage. The pilot reported that there were no mechanical failures or malfunctions, and noted that the fluctuating oil pressure gauge was likely due to the sending unit location on the engine and associated vibration or a loose ground connection.