"hijacked" malaysia confirms, page-248

You don't read the threads after posting a comment by the looks of it.

" the Greek plane, which had its oxygen turned off by the maintenance crew, who then forgot to turn it back on, leading to oxygen deprivation (although this seems very unlikely, the fact that it flew for some considerable time without any contact with ground control gives this scenario a degree of credence);"

While semi, or totally unconscious, the pilots turned the transponder off, and performed a few changes of heading as well?

The maintenance crew can't forget to turn the oxygen back on, as they don't turn it OFF in the first place.
Oxygen is only used should there be a loss in cabin pressure, then the oxygen masks will automatically be available for use. The pilot could most likely trigger the oxygen masks release as well.