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.
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