Is it possible to have both parents with o positive bloodtype, but their child have o negative blood type?

Yes. The rh factor is inherited through separate genetic means from the blood type. The rh factor is a dominant gene, meaning only one gene is needed to cause expression of the factor. So two rh + parents can have a rh+/rh- gene pattern and have a 1/4 chance with every conception of having an rh-/rh- baby, 1/4 chance of rh+/rh+ baby and 1/2 of carrier rh+/rh-.
See below. The transmission of the rhesus factor is not directly related to the ABO type. This means that the child could end up being negative.