I am blood type o-, my sister is o+, my mother is a+. do my sister and i have the same father?
Your mother would have to be genotype A/O +/-, phenotype A+
Your father would have to be at least heterozygous for O and Rh-, e.g. B/O +/- (phenotype B+) but capable of yielding offspring that are O- and O+.
This is pretty much answered by the prior responses, but O is the most recessive blood type and RH- is recessive. Therefore, the blood types of parents can easily differ from O-.
The above presumes blood types are purely Mendelson traits, but the presence of a genetic trait does not indicate parentage; the absense does.
More directly answering your question, my understanding is that, for genetic analysis purposes, O type blood can be regarded as an absense of A an B traits, or a "null" trait. RH- is definitely the absense of the RH factor.
This sort of thing is part of the basis of the Neanderthal Hsn investigations. It is thought that O and RH- are characteristic of Neanderthal, but ethnic groups not associated with a high percentage of Neanderthal genetic heritage nevertheless have a high O rate. (I'm not sure about Rh, but this may be similar.)