The pilot reported that, during takeoff for the accident flight, which was the second flight of the day, he saw that the oil pressure was falling and realized it was likely due to his failure to install the engine oil filler cap after checking the oil during the first flight of the day. He did not conduct a preflight checklist before the accident flight. He subsequently made a precautionary landing at a nearby airport. During the landing, the airplane bounced, and the landing gear collapsed. The airplane bounced again, skidded off the right side of the runway, and ground looped. 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.