Air carrier technician reported they forgot to install the oil cap after servicing the number 1 engine on the ERJ175.

Narrative

I put the aircraft out of service to do a oil service and tire pressure check of an aircraft and once I finished servicing the oil on engine 1 a pilot approached me and alerted me that the galley closet door wasn't sliding shut I brought my tools to the truck and proceeded to clean the galley closet tracks and close the door I left the aircraft and grabbed my tools from the truck and proceeded to finish servicing the oil on engine 2 and finished checking the last 2 tires (MLG3&4) pressure; I assumed I had reattached the oils service cap and closed the service doer for #1 engine and did not go back to make sure. Later I heard about the delay in ZZZ from a coworker and while cleaning my maintenance vehicle at the end of my shift I discovered a engine oil cap which I must have left there while servicing aircraft X due to the engine oil cap lanyard being detached Cause: the pilots interruption to alert me about the write up concerning the cabin interior item which became more urgent to complete due to passengers boarding; this disrupted my work flowSuggestions: Verify all maintenance actions are complied with prior to signing off; do a walk around of the aircraft after completing all maintenance actions