Why is my blood group O -ve?
Humans are diploid organism. So humans have two copies of every chromosome and gene. 

When a person has B blood. We mean that the blood cells of that person expresses  the B antigen. 

To produce the B antigen you will need at least one allele for the B antigen. Hence the B antigen phenotype can have a genotype of BB or BO. 

Now if two people have the genotype of BO have children, a punnet square will show they they have a 25% of having a child with BB genotype, 50% of having a child with BO genotype and 25% of having a child with OO genotype. 

Phenotypically, it means that there is 75% chance of having a child with B blood. And 25% chance of a child with O blood.

A similar system also occurs with rhesus +/- system. A person with +ve blood can have a genotype of ++ or +-. If two people with the genotype of +- have children together, there is a 25% chance that their child will have -- genotype and be rhesus -ve.

The two systems work together. So two people with genotype of BO blood type and +- rhesus antigen having children together, they can have a child with O blood and rhesus negative with a probability of 1/16 (6.25%)