Question

My Dad was blood type O Rh+ and my mom is blood type B. Is there a way I can tell what I am or might be without getting my blood tested?

Answer 1

Not conclusively; you could be either O or B and you could be either Rh+ or Rh-.

Explanation:

Genetically your father will be O-O (total absence of alleles/gene sequences for A or B).
Genetically your mother could be either B-B or B-O (possessing two or one allele/gene sequence for B).

Similarly, for the Rh factor, genetically your father could be either D-D (D being the most common Rhesus antigen) or D-d (d standing for the missing antigen).
Since you have not mentioned whether your mother is Rh+ or Rh-, genetically she could be D-D, D-d, or d-d.

Since you get one set of chromosomes from your father and one from your mother, you could be O-O (Blood group O) or O-B (Blood Group B), with chances of O-O being 50% if your mother is B-B and increasing to 75% if your mother is genetically B-O.

Your Rhesus factor could be D-D, D-d, d-D, or d-d (the first three being Rh+, the last being Rh-, the chances varying according to your mother's Rh status and the genetic variables.)

Better to get a blood test done. :)