Can O-positive And A-positive Have A Child With O-negative?

Yes, it would be possible. If you have an O+ parent and an A+ parent the possible blood types for their offspring would be:

A+
A-
O+
O-

If you want to calculate possible offspring blood types depending on the parents blood types, you can go here.

The ABo system and the rhesus group are two distinct groups.    As both these parents are positive ,the only way they can have a neg. Child is if both these parents are carriers for neg (To be neg you need two copies),which means that neither of these parents are pure breeding for their pos.groupings.   For a child from these parents to also be o (as well as neg. ) the A parent also needs to be a carrier for the o allele.   O is the recessive condition and neg is the recessive condition too.

O positive plus A positive=o negative?

Yes, a 25 percent chance of having a child with type o negative

The negative rH is possible if they are carriers of the negative rH factor. Both of my parents are positive while my maternal grandmother was negative. I and my little brother are both A- while my mom is O+ and my dad is A+. However, I think that since O means there are no "proteins" (or whatever it is that makes O's A's and B's) the dominant gene would be the A so the child could be born A-.

The mom is an o positive and the dad is o negative