Could you exercise on oxygen if you have emphysema and your o2 sat dip to low 90's?

Yes. As long as your o2 sats stay at 90% or above, your cells will get plenty of oxygen, and you can exercise safely on oxygen. There is very little increased benefit with o2 sats greater than 90%. The key is to stay above 90%.
Rehab.., One way to exercise is to participate in pulmonary rehab, where you can exercise with close monitoring and learn the exercise you can tolerate, on oxygen with copd. Exercise has been shown to be beneficial in COPD so see your doctor so rehab can be prescribed and you can exercise safely.
Yes. Exercise is very beneficial to patients with copd/emphysema. Exercising can be done on oxygen as long as your oxygen saturations are greater than 88 percent.