This is the third installment of the Galactic O-Star Spectroscopic Survey (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. 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, 6 of which are of O+O type and 5 of O+B type, and an additional new tripled-lined spectroscopic binary 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.

Cone search capability for table J/ApJS/224/4/table4 (Spectral reclassifications for stars already present in Papers I and II (Cat. III/274))

Cone search capability for table J/ApJS/224/4/table6 (Spectral classifications for late-type stars erroneously classified as O stars)

Cone search capability for table J/ApJS/224/4/table5 (Spectral classifications for new GOSSS stars)

Cone search capability for table J/ApJS/224/4/table3 (Stars in Papers I+II with recently discovered bright companions unresolved in the GOSSS spectra)