There are sufficient videos of people in hypobaric chambers clearly demonstrating their gradual slip from fully aware to becoming incapacitated in managing even simple skills. The worst part is the general feeling that everything is OK even when their performance is getting far worse. They do seem to recognize they are having trouble but not in a way that indicates the fear they should be having at being deprived of oxygen.
In a short time Kimberly says she feels like she needs oxygen but is unable to figure out how to put the mask back on her face even though it is already nearly in place.
Less amusing:
Samuel exits the test and soon another is pleased to simply be able to hold his pencil
The main take-away for me is to not dawdle when the masks drop. No "one last thing' or "just a second."
I expect that for commercial pilots it's also, if the other guy is acting stupid, put your mask on right now and then figure out the other guy; don't stop to ask "Are you OK?"