  The ab initio $GW$ method is considered as the most accurate approach for
calculating the band gaps of semiconductors and insulators. Yet its application
to transition metal oxides (TMOs) has been hindered by the failure of
traditional approximations developed for conventional semiconductors. In this
work, we examine the effects of these approximations on the values of band gaps
for ZnO, Cu$_2$O, and TiO$_2$. In particular, we explore the origin of the
differences between the two widely used plasmon-pole models. Based on the
comparison of our results with the experimental data and previously published
calculations, we discuss which approximations are suitable for TMOs and why.
