  The apparent lack of massive O-type stars near the zero-age main sequence (at
ages < 2 Myr) is a topic widely discussed. Different explanations for this
elusive detection have been proposed, but no firm conclusions have been reached
yet. We reassess this empirical result benefiting from the high-quality
spectroscopic observations of >400 Galactic O-type stars gathered by the IACOB
and OWN surveys. We used temperatures and gravities from a iacob-gbat/fastwind
spectroscopic analysis to locate our sample in the Kiel and spectroscopic HR
diagrams. We evaluated the completeness of our sample of stars, observational
biases using information from the Galactic O star catalog (GOSC), systematics
of our methodology, and compare with other recent studies using smaller samples
of Galactic O-type stars. We base our discussion on the spectroscopic HR
diagram to avoid the use of uncertain distances. We performed a detailed study
of the young cluster Trumpler-14 as an example of how Gaia cluster distances
can help to construct the associated classical HR diagram. The apparent lack of
massive O-type stars near the ZAMS with masses between 30 and 70 Msol persist
even when spectroscopic results from a large, non-biased sample of stars are
used. We do not find correlation between the dearth of stars and observational
biases, limitations of our methodology, or the use of spectroscopic HR diagram
instead of the classical one. Investigating the efficiency of mass accretion
during the formation process we conclude that an adjustment of the accretion
rate towards lower values could reconcile the hotter boundary of detected
O-type stars and the theoretical birthline. Last, we discuss that the presence
of a small sample of O2-O3.5 stars found closer to the ZAMS might be explained
taking into account non-standard star evolution (e.g. binary interaction,
mergers, or homogeneous evolution).
