  We use synthetic model spectra to investigate the potential of
near-ultraviolet (3000-4050 \r{A}) observations of massive O-type stars. We
highlight the He I $\lambda$3188 and He II $\lambda$3203 pair as a potential
temperature diagnostic in this range, supported by estimates of gravity using
the high Balmer series lines. The near-ultraviolet also contains important
metallic lines for determinations of chemical abundances (oxygen in particular)
and estimates of projected rotational velocities for O-type spectra. Using the
model spectra we present performance estimates for observations of
extragalactic massive stars with the Cassegrain U-Band Efficient Spectrograph
(CUBES) now in construction for the Very Large Telescope. The high efficiency
of CUBES will open-up exciting new possibilities in the study of massive stars
in external galaxies. For instance, CUBES will provide new insights into the
physical properties of O-type stars, including oxygen abundances, in metal-poor
irregular galaxies at ~1 Mpc from integrations of just 2-3 hrs. Moreover, CUBES
will bring quantitative spectroscopy of more distant targets within reach for
the first time, such as the O-type star (V~21.5 mag) in Leo P (at 1.6 Mpc) in
only half a night of observations.
