Large Transport
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Communication Breakdown
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Landed As Precaution; Flight Crew Diverted; Flight Crew Returned To Departure Airport; Flight Crew Requested ATC Assistance / Clarification; General Flight Cancelled / Delayed
Aircraft
I was the pilot monitoring when we originally departed Hawaii on time for the flight back to the mainland.  After leveling at cruise and checking in with SFO radio (and making our first position report) I asked the captain if we could set up a bathroom break.  During that process the Captain's mask became detached from the oxygen system and he was forced to turn off the crew oxygen system supply due to uncontrolled flow.  In accordance with ETOPS requirements we initiated a turn around back to Hawaii. During that process; we had very difficult communication with SFO (1x1) and very little communication with Honolulu on VHF.  We were unable to clearly ascertain our altitude clearance; so after discussing it we elected to vertically offset.  When we confirmed the clearance; we returned to our original altitude and returned to Hawaii uneventfully.
Air carrier First Officer reported oxygen system failure associated with the Captain's mask during cruise. Flight crew elected to return to the departure airport.