What does it mean if your oxygen is at 91?

A little low: Assuming you mean a "pulse ox" result, than 91% means that your are getting 91% oxygen saturation at the tips of your fingers. Normal is 97-100%. As a gauge, medicare guidelines will pay for oxygen if under 88%. If you have asthma or copd, this may be baseline for you? Check with your dr and follow up if your pule ox remains in the low 90's as you want to assure this is followed up on.