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.)