In my previous job, I spent most of my time flying between 20,000 and 23,000 in unpressurized airplanes (for 5 to 6 hour legs). So did my collegues.
It is hard to eat a meal with the O2 mask on. It is not hard to breathe-- you don't feel like you are drowning, but you are feeling the effects. At 23,000, I would feel the need to slap the mask back on my face within a minute and it felt so good when I did. Some people could last longer. I noticed that this would vary for me from day to day-- depending on how well I slept, how tired I was, etc.
The biggest problem was not using enough O2. Some guys were really stingy. Others wanted to show how little O2 they could use. One bonehead actually bragged that he did a whole 2½ hour flight without O2-- but I've always wondered about this guy. The Company did little to help, and actually promoted this concept as a cost savings.
One guy came to me (older pilot) and told me he was getting DCS. He was convinced of it. I told him to try cranking the O2 open. No way, he was suffering from DCS. I insisted. He followed my suggestion, and guess what-- he was fine! We had another guy, first flight on the line, who used so little O2 (around 300 psi for a 5 hour flight) he felt sooooooo sick when he landed that he swore he couldn't handle the high altitude, that he was getting DCS also. He looked like he was at death's doorstep. It took some time to convince him that it wasn't DCS, that he needed to crank the O2. Guess what, next time I saw him he had a big smile on his face.
The problem with hypoxia is you can get it using O2, if you don't use enough. Most guys said they were not suffering hypoxia, but after prompting them to crank the flow up they realized how much better they were were feeling and that they were actually somewhat hypoxic. Unlike being pregnant, hypoxic is not a situation where you "are" or "you are not".
Remember, the biggest consumer of O2 is your brain. As a result, before you get the tingling in your fingers or legs, your brain is somewhat impared. It actually keeps you from realizing how degraded your skills are. Kinda like people who feel they are just as good after a drink-- I don't feel dizzy-- I am not affected yet!