  Massive stars are predominantly found in binaries and higher order multiples.
While the period and eccentricity distributions of OB stars are now well
established across different metallicity regimes, the determination of
mass-ratios has been mostly limited to double-lined spectroscopic binaries. As
a consequence, the mass-ratio distribution remains subject to significant
uncertainties. Open questions include the shape and extent of the companion
mass-function towards its low-mass end and the nature of undetected companions
in single-lined spectroscopic binaries. In this contribution, we present the
results of a large and systematic analysis of a sample of over 80 single-lined
O-type spectroscopic binaries (SB1s) in the Milky Way and in the Large
Magellanic Cloud (LMC). We report on the developed methodology, the constraints
obtained on the nature of SB1 companions, the distribution of O star
mass-ratios at LMC metallicity and the occurrence of quiescent OB+black hole
binaries.
