I did a sleep study and they confirmed that I needed oxygen during sleep because my O2 saturation average for the night was 86%. I have been on night oxygen for less than a year and don't notice any difference in terms of my energy when awake, but still I understand why it's needed (mostly for the long term condition of the heart).
I need oxygen during exercise and this need is very clear to me. My O2 levels go down immediately after I start working out, so I notice a difference. I don't mind using it because I exercise 3x a week at a physiotherapy clinic, so it's a "safe" environment and I know how badly I need exercise.
My O2 saturation at rest is, at best, 95%, so I don't need oxygen while at rest. However, I am short of breath as soon as I go up stairs, walk fast (or even normal), carry things, etc etc....and my O2 comes down to the 80s. My doctor however is not asking me to use oxygen during my day to day activities though. The reason might be that even though I get very tired during activity, I have time to pace myself and I can always stop and rest, take it easy etc, because I am not working and not usually in a rush.
So to summarize, I believe that we should be on oxygen when our saturation is around 88% or below and it STAYS THERE, not coming back up to the 90s. This is just my experience.