How could the Helios 522 passengers have survived so long without oxygen?
To add to Daniele's answer, from the final report:


  The  forensic  report  concluded  that  the  aircraft  occupants  had  heart  function  during  the  impact.    The  report  noted  that  this
    did  not  necessarily  imply  that  they  were alert.    The  report  further  estimated  that  they  were  in  deep  non-reversible  coma  due  to 
  their prolonged exposure (over 2.5 h) to the high hypoxic environment. 


So, again, saying that they were alive does not mean that they were well.