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.

O positive plus A positive=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

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