Beech Bonanza pilot reported observing an oil leak on top of the engine cowling during climb. The pilot returned to the departure airport and landed safely.

Narrative

On departure off of Runway XXR at ZZZ I noticed oil coming from the dip stick/oil filler access door on the top of the cowling. I asked Departure for vectors back to ZZZ because of the unusual oil seepage from the access door. ZZZ Departure began to vector me back to Runway XXR; then offered me an immediate return to Runway YYR. The wind was 360 at 5; a direct crosswind. Initially I declined; but then accepted the return to YYR to minimize oil loss and the potential of my windscreen becoming obscured with oil. I was too high for a safe landing on Runway YYR; so I asked for a 360 degree turn to lose altitude and it was approved. At this point I was switched over to ZZZ Tower and was cleared to land. The landing was without incident. I taxied to parking and after raising the cowling; I discovered I had failed to re-install the oil cap after adding oil during the preflight inspection. In retrospect; I realized that I had several distractions while adding oil during preflight; which lead to my failure to re-install the oil cap. I re-installed the oil cap which attached firmly and secure and cleaned up as much oil as possible. I only found oil on the top of the engine. No oil was leaking on to the ground. I rechecked the oil quantity; it showed 10 quarts remaining; which is the maximum oil quantity recommended. Based on the amount of oil cleaned up; I would expect that no more than several tablespoons of oil had been lost. The oil pressure was normal and consistent after engine start and during a long taxi and preflight engine run up. I elected to take-off. The departure and flight were without incident.