Cumulative histograms of the nominal relative error in parallax of increasing large samples of Galactic O-type stars with available parallaxes following the Tycho-2 (Hog et al. 2000) astrometric catalogs, and the various Gaia data releases, including the Gaia-TGAS solution (Michalik et al. 2015). For each catalog we indicate the total number of known stars of this type with available entries, along with those in which a relative error in parallax <10% was achieved (see also vertical line).