  This is the third installment of GOSSS, a massive spectroscopic survey of
Galactic O stars, based on new homogeneous, high signal-to-noise ratio, R~2500
digital observations selected from the Galactic O-Star Catalog (GOSC). In this
paper we present 142 additional stellar systems with O stars from both
hemispheres, bringing the total of O-type systems published within the project
to 590. Among the new objects there are 20 new O stars. We also identify 11 new
double-lined spectroscopic binaries (SB2s), of which 6 are of O+O type and 5 of
O+B type, and an additional new tripled-lined spectroscopic binary (SB3) of
O+O+B type. We also revise some of the previous GOSSS classifications, present
some egregious examples of stars erroneously classified as O-type in the past,
introduce the use of luminosity class IV at spectral types O4-O5.5, and adapt
the classification scheme to the work of Arias et al. (2016).
