  A significant fraction of massive stars in the Milky Way and other galaxies
are located far from star clusters. It is known that some of these stars are
runaways and therefore most likely were formed in embedded clusters and then
ejected into the field because of dynamical few-body interactions or
binary-supernova explosions. However, there exists a group of field O stars
whose runaway status is difficult to prove via direct proper motion
measurements or whose low space velocities and/or young ages appear to be
incompatible with their large separation from known star clusters. The
existence of this group led some authors to believe that field O stars can form
in situ. In this paper, we examine the runaway status of the best candidates
for isolated formation of massive stars in the Milky Way and the Magellanic
Clouds by searching for bow shocks around them, by using the new reduction of
the Hipparcos data, and by searching for stellar systems from which they could
originate within their lifetimes. We show that most of the known O stars
thought to have formed in isolation are instead very likely runaways. We show
also that the field must contain a population of O stars whose low space
velocities and/or young ages are in apparent contradiction with the large
separation of these stars from their parent clusters and/or the ages of these
clusters. These stars (the descendants of runaway massive binaries) cannot be
traced back to their parent clusters and therefore can be mistakenly considered
as having formed in situ. We argue also that some field O stars could be
detected in optical wavelengths only because they are runaways, while their
cousins residing in the deeply embedded parent clusters might still remain
totally obscured. The main conclusion of our study is that there is no
significant evidence whatsoever in support of the in situ proposal on the
origin of massive stars.
