  In two papers we try to confirm that all Galactic high-mass stars are formed
in a cluster environment, by excluding that O-type stars found in the Galactic
field actually formed there. In de Wit et al. (2004) we presented deep K-band
imaging of 5 arcmin fields centred on 43 massive O-type field stars that
revealed that the large majority of these objects are single objects. In this
contribution we explore the possibility that the field O stars are dynamically
ejected from young clusters, by investigating their peculiar space velocity
distribution, their distance from the Galactic plane, and their spatial
vicinity to known young stellar clusters. We (re-)identify 22 field O-type
stars as candidate runaway OB-stars. The statistics show that ~4% of all O-type
stars with V < 8 can be considered as formed outside a cluster environment.
Most are spectroscopically single objects, some are visual binaries. The
derived percentage for O-type stars that form isolated in the field based on
our statistical analyses is in agreement with what is expected from
calculations adopting a universal cluster richness distribution with power
index of beta = 1.7, assuming that the cluster richness distribution is
continuous down to the smallest clusters containing one single star.
