O Vz stars are defined as those O-type stars displaying HeII 4686 absorption line stronger than other He line in the blue spectral region. These stars have been hypothesized on spectroscopic grounds to be O-type stars on or close to the Zero-Age-Main-Sequence (ZAMS), also having weaker winds than normal O V stars. We have tested these hypothesis by performing a quantitative spectroscopic analysis of a sample of 37 O Vz (plus 47 O V stars used for comparison) discovered by the VLT-FLAMES Tarantula Survey (VFTS) in the 30 Doradus region within the Large Magellanic Cloud. To this aim, we have used the IACOB grid based automatic tool, a fast, accurate and objective automatic method to determine the stellar and wind parameters of large samples of O stars in a reasonable amount of time. A first inspection of the resulting physical properties of the O Vz stars in 30 Dor is presented here. The main outcome from our quantitative spectroscopic analysis is that the O Vz phenomenon is a natural consecuence of the combination of stellar parameters and it does not reflect a differencial characteristic in age, gravity, proximity to ZAMS or wind-strength, when compared to O V stars from the same region.