B787-900
Situational Awareness; Training / Qualification
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Departure Airport
Human Factors; Procedure
I believed [I] closed cap and tab locked the right engine oil fill cap but [it] wasn't locked. Then right engine [started] losing oil and pressure.[I recommend] to confirm cap closed and locked before closing service panel or panel can't close unless cap is locked.
Maintenance Technician reported a 787-900 engine oil fill cap was not closed and locked before departure.