They like to keep it above 97% ideally and will put people with anything below this on oxygen. 15% is pretty incredible, I bet the doctors don't see that everyday. I'm glad you are ok! Do you know how you got into this state?
People are confusing the oxygen content of the blood and the oxygen saturation. These are between the following numbers 15 - 23% for content and 95 to 100% for saturation. The first figure tells us how much oxygen is present in the blood, and the second is how much of that oxygen is being carried by the red blood cells.
Oxygen saturation is routinely taken via the latest bood pressure machines, these also record pulse rates too, the normal values are between 97% and 100% In the hospital that I work at, only sustained values below 93% would be considered for oxygen therapy and this has it's own risks. You can increase your oxygen saturation reading immediately by taking a deep breath and holding it. Judy