  Around 4% of O-stars are observed in apparent isolation, with no associated
cluster, and no indication of having been ejected from a nearby cluster. We
define an isolated O-star as a star > 17.5 M_\odot in a cluster with total mass
<100 M_\odot which contains no other massive (>10 M_\odot) stars. We show that
the fraction of apparently isolated O-stars is reproduced when stars are
sampled (randomly) from a standard initial mass function and a standard cluster
mass function of the form N(M) \propto M^-2.
  This result is difficult to reconcile with the idea that there is a
fundamental relationship between the mass of a cluster and the mass of the most
massive star in that cluster. We suggest that such a relationship is a typical
result of star formation in clusters, and that `isolated O-stars' are low-mass
clusters in which massive stars have been able to form.
