Contacting a local hospital and asking them the percentage of the population that has blood type O will generate different results.
The factors that we have to consider why there is differing results are:
1) location of the hospital
2) nationality of their patients
3) number of their patients
I am assuming that the population that question is referring to is the number of patients in the local hospital. The bigger the population, the smaller the effect a unit has on the whole and vice versa.
I read an article that states that 37% of the U.S. population has O+ blood type. These people are usually of Hispanic descent or some Asian descent. So, if a hospital is in a locality that has a majority of Hispanic or Asian patients, its percentage will be higher than a hospital that is located in a Caucasian-populated area.
Aside from Type O+ (most common), blood types also include: O-, A+, A-, B+, B-, AB+, and AB- (rarest blood type)