Can a opositive and a opositive make a abnegative baby?

No, two parents with an O positive blood type cannot have a child with an AB negative blood type. The ABO blood group system requires one parent to contribute an A or B allele for an AB child, which an O parent cannot provide. Additionally, for a child to be Rh negative (like AB negative), at least one parent must carry an Rh negative allele, which is also not possible if both parents are Rh positive.