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The apparent lack of massive O-type stars near the zero-age main sequence (atages < 2 Myr) is a topic widely discussed. Different explanations for thiselusive detection have been proposed, but no firm conclusions have been reachedyet. We reassess this empirical result benefiting from the high-qualityspectroscopic observations of >400 Galactic O-type stars gathered by the IACOBand OWN surveys. We used temperatures and gravities from a iacob-gbat/fastwindspectroscopic analysis to locate our sample in the Kiel and spectroscopic HRdiagrams. We evaluated the completeness of our sample of stars, observationalbiases using information from the Galactic O star catalog (GOSC), systematicsof our methodology, and compare with other recent studies using smaller samplesof Galactic O-type stars. We base our discussion on the spectroscopic HRdiagram to avoid the use of uncertain distances. We performed a detailed studyof the young cluster Trumpler-14 as an example of how Gaia cluster distancescan help to construct the associated classical HR diagram. The apparent lack ofmassive O-type stars near the ZAMS with masses between 30 and 70 Msol persisteven when spectroscopic results from a large, non-biased sample of stars areused. We do not find correlation between the dearth of stars and observationalbiases, limitations of our methodology, or the use of spectroscopic HR diagraminstead of the classical one. Investigating the efficiency of mass accretionduring the formation process we conclude that an adjustment of the accretionrate towards lower values could reconcile the hotter boundary of detectedO-type stars and the theoretical birthline. Last, we discuss that the presenceof a small sample of O2-O3.5 stars found closer to the ZAMS might be explainedtaking into account non-standard star evolution (e.g. binary interaction,mergers, or homogeneous evolution).

The IACOB project. VI. On the elusive detection of massive O-type stars close to the ZAMS