Human blood is very complex with entire fields of study and medical specialization dedicated to it. The two most important things that distinguish your blood are its ABO Group and its Rh Factor. Together these make up your blood type, O- in your case.
The 15% number you were given was probably for your Rh factor since about 15% of the population is Rh-, with the other 85% being Rh+. The Red Cross provides a breakdown of the U.S. population by blood type, which also depends on your race. According to their numbers 8% of caucasions in the U.S. have a blood type of O-.
http://www.redcrossblood.org/learn-a...od/blood-types
In addition to being very important if you need a blood transfusion, your blood type matters when you become pregnant. Specifically, a woman with one of the Rh- blood types who carries a baby with an Rh+ blood type needs to understand the risks of interaction between her blood and the baby's blood and the implications for a future pregnancy. As long as you get good prenatal care, your doctor will educate you about this and take preventive measures if necessary. You can read more about it here:
https://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0010419/