B767-300 Captain reported concerns over the crew O2 being sufficient pressure to sustain the flight crew during a diversion.

Narrative

I am submitting this report concerning Aircraft X flown on Day 1 from ZZZ-ZZZZZ wherein the Critical Fuel Point was ETP1 with the Alternate airports of ZZZ1 and ZZZZZ1 with a diversion time of four hour and two minutes. The flight was dispatched under ETOPS Rule X. The Captain contacted flight control with concerns that the CFP was ETP1 with a diversion time well in excess of 180 minutes and whether or not the aircraft had enough oxygen to sustain a crew of four in the event of a Non-Normal emergency requiring oxygen for four plus hours of flight time. We were not able to directly determine the minimum amount of oxygen required due to the AOM chart only going to 180 minutes however we were informed by a system expert that the same variances for the actual diversion time exceeding 180 minutes would apply to the required oxygen limits. flight control suggested that we could remove the P3 crew member although the Captain stated that he was still unable to determine how much Oxygen would be required for any number of crew members at such flight time due to AOM table limitations an as such was seeking assistance. With the assurance that the Oxygen and ETOPS planning were acceptable the flight was conducted without incident. Of note; the Oxygen pressure at departure from ZZZ was 1540 pounds with a reported temperature of 20 Degrees C and was at 1510 pounds on arrival at ZZZZZ with a reported temperature of 20 Degrees C.