  The stellar IMF has been found to be an invariant Salpeter power-law
(alpha=2.35) above about 1 Msun, but at the same time a massive star typically
has more than one companion. This constrains the possible formation scenarios
of massive stars, but also implies that the true, binary-star corrected stellar
IMF could be significantly steeper than Salpeter, alpha>2.7. A significant
fraction of all OB stars are found relatively far from potential birth sites
which is most probably a result of dynamical ejections from cores of
binary-rich star clusters. Such cores form rapidly due to dynamical mass
segregation, or they are primordial. Probably all OB stars thus form in stellar
clusters together with low-mass stars, and they have a rather devastating
effect on the embedded cluster by rapidly driving out the remaining gas leaving
expanding OB associations and bound star clusters. The distributed population
of OB stars has a measured IMF with alpha about 4, which however, does not
necessarily constitute a different physical mode for isolated star formation. A
steep field-star IMF is obtained naturally because stars form in clusters which
are distributed according to a power-law cluster mass function.
