  We analyze the Higgs-boson masses and mixing matrices in the NMSSM based on
an on-shell (OS) renormalization of the gauge-boson and Higgs-boson masses and
the parameters of the top/scalar top sector. We compare the implementation of
the OS calculations in the codes NMSSMCALC and NMSSM-FeynHiggs up to
$\mathcal{({\alpha}_t{\alpha}_s)}$. We identify the sources of discrepancies at
the one- and at the two-loop level. Finally we compare the OS and
$\overline{\text{DR}}$ evaluation as implemented in NMSSMCALC. The results are
important ingredients for an estimate of the theoretical precision of
Higgs-boson mass calculations in the NMSSM.
