AS 365 Eurocopter Technician reported that they inadvertently forgot to reinstall the engine oil cap after servicing.

Narrative

I came in that morning and reviewed the due report for what maintenance needed to be completed. After taking approximately 10 minutes to clear my head of distractions I went out to the aircraft to perform the needed maintenance for the day. During the airworthiness check I noticed the engine oil was a little low so I proceeded to get a quart of oil to service the #2 engine. After servicing I went into the office to sign off all of my maintenance for the day and took the log book to the pilot and returned to the maintenance office to complete the Maintenance Software entries and finish a few non aircraft related tasks before leaving to assist with packing equipment at the moraine base to prepare for the end of contract. At approximately XA:40 the pilot called me to tell me that they were on top of the hospital with oil all over the transmission deck and that the oil cap was not on the tank. That was the moment I realized I allowed my personal distractions to creep in and take my mind off the task at hand and forgot to reinstall the cap on the oil tank. I then grabbed more oil and rags and drove to the hospital to refill the tank and clean the aircraft.Suggestions: Take as much time as necessary to ensure mind is on task at hand. If outside/personal distractions are affecting person performing maintenance contact someone for assistance or take a break until able to refocus on work at hand.