The pilot in the tailwheel-equipped airplane reported that, during post-flight maintenance he left the oil filler cap off. Subsequently, during takeoff, he realized that the oil filler cap was not installed. The pilot noticed a decrease in oil pressure and made a precautionary landing at a nearby airport. During landing, the airplane landed hard, bounced, and the landing gear collapsed. The airplane skidded off the right side of the runway, and ground looped to the right. The left wing struck the ground before the airplane stopped.
The airplane sustained substantial damage to the main rotor blades.
The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.