Can a child have a negative blood type if both of her parents are o positve?

Yes: Each parent has two rh genes. It only takes one positive gene to make a person rh positive, so each parent can have a positive and a negative rh gene. Both genes have to be rh negative for someone to be rh negative. Since each parent only contributes one gene to child, the child can get a negative gene from each parent and therefore be rh negative.

Maybe: Rhesus transmission (+ or -) is not a clear as the ABO transmission. It is also separate from the genetics for "blood type". This is purely a result of the genetic expression of each parent, and this may be different when their genes are combined in a new child.