O2 level needs depends on what you are doing at the time and your health conditions, lung capabilities. The more you move the more O2 required. And when we sleep our breathing is lower, OK for healthy people but compromised people need help. For example, just sitting on the sofa, my O2 is fine, but for exercise and sleeping I require oxygen supplement.
My diagnosis includes at least 3 reasons for shortness of breath: 1) interstitial pulmonary fibrosis as a previous smoker 2) Episode of radiation pneumonitis with remaining scar tissue 3) Possible pulmonary hypertension, reduced right ventricular function.