Recollections from military training! At 10000', the partial pressure of O2 is 1/2 that at sea level which is the point at which you start needing to increase the level of O2, so a military mask will start to give an O2 airmix to solve this problem. More clever stuff happens higher up.
It is ok to operate at this 10000 level indefinitely but above that not so, which is why the CABIN ALT warning goes off at 10000' cabin alt.
Problems with concentration come not from the reduced O2 in a properly functioning cockpit but from a myriad of factors - time zone changes, sleep patterns, diet, temperature, vibration, noise, under arousal among others and some or all of which exist in aviation and other careers. Certainly in a short-term office based job poor eating habits, insufficient sleep, repetitive work, higher temperatures and others made concentration very difficult.