  Our goal is to provide a quantification of several spectral classification
criteria for O stars. We collected high-spectral resolution spectra of 105
Galactic O-type stars from various archives. We measured equivalent widths of
classification lines. We defined average values of classification criteria for
given spectral types and luminosity classes. We find that the ratio HeI 4471 to
HeII 4542 well matches the published ratios for spectral types. We have
quantified equivalent width ratios of helium and silicon lines among O8-O9.7
stars to refine spectral class typing in this spectral range. We present
quantitative criteria to separate between luminosity class V, IV-III-II
(grouped), and I among O3-O8.5 stars, mainly based on the strength of HeII
4686. We find that these criteria also define very well the f, (f), and ((f))
classes for O3-O7.5 stars. Among O9-O9.7 stars we quantify the ratios of HeII
4686 to HeI 4713 and SiIV 4089 to HeI 4026 for all luminosity classes. The
tabulated values of the classification criteria should help classify any new
O-type stars. The final step of the classification process should rely on a
direct comparison to standard stars of the assigned spectral type and
luminosity class.
