testing oximeter

Can you test an oximeter by holding your breath? I held my breath for 30 seconds and it didn't change. Stayed at 97

It takes longer than 30 seconds to see much of a change in O2 levels in people with healthy lungs.
You might see a change if you held it for 30 seconds then breathed a few breaths and then held it again for 30 seconds and breathed a few breaths and kept repeating the cycle.
Otherwise you would most likely have to hold your breath a lot longer in one setting to see a change and even then the change is slow.