  The modern-era spectral classification of O-stars relies on either the
Walborn or the Conti-Mathys scheme. Since both of these approaches have been
developed using low-quality photographic data, their application to
high-quality digital data might not be straightforward and be hampered by
problems and complications that have not yet been appreciated. Using
high-resolution spectra obtained with the ESO/MPG 2.2\,m telescope in La Silla
and following the premises of the Walborn and Conti classification schemes, we
determined the spectral types and luminosity classes of 19 Galactic O-type
stars and compared them to those attributed by Walborn and Mathys based on
low-quality data. Our analysis reveals that the morphological spectral types
assigned using high-resolution data are systematically later (by up to 1.5
subtypes) then those attributed by Walborn. By means of line-profile
simulations, we show that part of this discrepancy is more likely caused by the
combined effect of stellar rotation and high spectral resolution on the depth
of helium lines used as spectral type indicators. In addition, we demonstrate
that at least for narrow-lined stars the "rotational effect" does not disappear
when the high-resolution spectra are degraded to the resolution of the Walborn
standards. We also find evidence of a systematic difference between our
high-resolution quantitative spectral types and those assigned by Mathys.
Rotation and spectral resolution are important third parameters in the spectral
classification of O-type stars. To obtain reliable spectral classes within the
Walborn approach, the unknown and the standard spectra must be compared at the
same resolution and \vsini. Owing to resolution effects, the Conti approach
might also need to be updated.
