Thank you for your explanation SIDSTAR !
... obviously I already know that all the procedures are the same in case of emergency descent, but I was doubtful about the one I mentioned in this thread. I didn't keep in mind cabin decompression routes that have to be used to ! ... Anyway I am perfectly aware that oxygen allows a good margine in terms of time, but Helios accident was caused by crew unawareness ... so they didn't realize of lack of cabin pressurization due to system failure ...
Best Regards,
captain87