Fatemeh:  Degassing only proportionally reduces the amount of O2 dissolved in medium solution, but does not reduce the O2% at all.  One needs to use a pre-mixed gas with lower portion of O2 among other gases for hypoxia experiment.

Thanks so much ke-Wei Zhao :)

Lets not forget that degassing will remove all gas's - including CO2, that you are very likely relying on to maintain optimal pH!

Not so simple!

You can try the Petaka bioreactor: http://www.neuromics.com/petaka-cell-culturing-systems