My mom has O+ blood, my dad is B+, and I'm A-. Is it true I'm the only one with Rh blood?
I am a bit confused by the wording of your question, but I can help you with most of it. The + symbol in blood types refers to having the Rh factor. The Rh factor is a type of protein on the surface of red blood cells. If a person has the Rh factor, they have positive (+) blood. If the Rh factor is not present, the person has negative (-) blood. Since you are A-, you do not have the Rh protein on your red blood cells.
I am at a loss as to how you have type A blood. Your mom is type O, which means that she can only donate the O type gene. Genotypically, her blood type is OO. She can only ever donate the O. I can't know your dad's genotype for sure, but because he is type B, he can either be a pure type B or a type B that carries the gene for O type blood. A pure B would genotypically be BB. If he carries the O gene, then his blood genotype is BO.
In either case, your dad can only donate an O or a B. Your mom can only ever donate an O. Neither parent can donate the gene for type A blood, so your blood type should not be type A.