GeorgeB wrote:

What would have been the effect if the O2 bottle for the cockpit had
been inadvertently filled with breathing air rather than O2?

They would have tested the system before the flight and found it
working. They would have put on their mask in flight and thought it
worked, slowly fading away without recognizing it. You may or may not
recognize hypoxia when you suspect it and really watch your body, you
definitely don't recognize it when you're unwary.

Stefan