Can being over weight cause decrease in 02 saturations?

Yes: Obesity can cause a form of restrictive lung disease because it is difficult to inflate the lungs to breathe.

Yes: But, it usually has to be very overt. Morbidly obese people will have restricted lung expansion, and overtime, may develop a disease known as pulmonary hypertension which in and of itself would lower their o2 saturation, but even prior to it, the restricted lung expansion can potentially lower o2 saturation.