The stellar IMF has been found to be an invariant Salpeter power-law
(alpha=2.35) above about 1 Mo, but at the same time a
massive star typically has more than one low-mass companion. This
constrains the possible formation scenarios of massive stars, but also
implies that the true, binary-star corrected stellar IMF should 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 ~ 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.