On the basis of the Galactic O Star Spectroscopic Survey (GOSSS), we present a detailed systematic investigation of the O Vz stars. The currently used spectral classification criteria are rediscussed, and the Vz phenomenon is recalibrated through the addition of a quantitative criterion based on the equivalent widths of the HeI{lambda}4471, HeII{lambda}4542, and HeII{lambda}4686 spectral lines. The GOSSS O Vz and O V populations resulting from the newly adopted spectral classification criteria are comparatively analyzed. The locations of the O Vz stars are probed, showing a concentration of the most extreme cases toward the youngest star-forming regions. The occurrence of the Vz spectral peculiarity in a solar-metallicity environment, as predicted by the FASTWIND code, is also investigated, confirming the importance of taking into account several processes for the correct interpretation of the phenomenon.

Cone search capability for table J/AJ/152/31/table3 (*Double-lined binary Stars in GOSSS)

Cone search capability for table J/AJ/152/31/single (Objects that are single lined in the GOSSS spectra and for which no evidence of binarity is known (table1), and objects that are single lined in the GOSSS spectra but are known to be spectroscopic binaries from high-resolution data (table 2))