  The question whether or not massive stars can form in isolation or only in
star clusters is of great importance for the theory of (massive) star-formation
as well as for the stellar initial mass function of whole galaxies
(IGIMF-theory). While a seemingly easy question it is rather difficult to
answer. Several physical processes (e.g. star-loss due to stellar dynamics or
gas expulsion) and observational limitations (e.g. dust obscuration of young
clusters, resolution) pose severe challenges to answer this question. In this
contribution we will present the current arguments in favour and against the
idea that all O stars form in clusters.
