How oxygen levels and ability to process air correlate, in obstructive and/or restrictive lung diseases

Can someone explain, and preferably people w lung disorders, why people, even w perhaps stage 4 lung disease, can still have a regular oxygen level, but still have difficulty breathing bc of obstruction and/or restriction of lungs? Ive heard that a lot of people will still have a proper oxygen level even w severe lung disorders, and as you may know, dont require oxygen until fev1 is at about 30 percent or lower. But they will still feel they have issues a breathing. So how do these peoples' lungs process oxygen thats so different, I guess thats a bad way to word it. And especially since each persons illness will be unique, but what im asking is... how much does oxygen have to do with ability to breathe, in this certain aspect? Also, how accurate is the finger reading on oxygen levels? How can peoples oxygen level be determined by just their finger? Ive only been to a pulmonary dr a few times but ive been diagnosed w asthma, both moderate obstructive and restrictive lung disease, chest wall disorder, intercostal neuralgia. Also a few other drs have remarked the because of scoliosis curvatures of the spine, my ribs/spine press on my lungs.

Is it that the body does pull enough oxygen from the bloodstream, but the lungs do not handle it correctly? And how would this occur? And especially if the carbon dioxide level is too high. Does co2 level get too high even if a persons oxygen level is normal? Ive also had a hospital record where the co2 was remarked as too high, but i think that my oxygen was regular. So how does this go on at the same time Tweet