This probably means very little but in the B777 in the QRH Unannunciated Checklist there is no guidence regarding crew action that may be associated with an EICAS message that says Crew Oxygen Low. Ditto for a Pax Oxygen Low message. This assumes that your 777 is equipped with a gaseous system as opposed to the 15 or 20 minute canisters. I guess it's fair to say that they will leave it up to the crew to act in the most prudent, safe manner to get the airpane and pax to their destination or alternate.