O negative blood is the universal blood type. Meaning that anybody can receive O negative blood. So it's not rare to have O negative blood, but it is in high demand as it can be transfused into patients no matter what blood type they are.
O- blood is rare! Only 7% of the population has it. It is correct that O- is universal donor but they can only receive O- in return. Having O+ is more common.
My understanding is the same as what Sarah mentioned above.
I would call Memorial Blood bank to get a good answer.
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