  We examine substructure and mass segregation in the massive OB association
Cygnus OB2 to better understand its initial conditions. Using a well understood
Chandra X-ray selected sample of young stars we find that Cyg OB2 exhibits
considerable physical substructure and has no evidence for mass segregation,
both indications that the association is not dynamically evolved. Combined with
previous kinematical studies we conclude that Cyg OB2 is dynamically very
young, and what we observe now is very close to its initial conditions: Cyg OB2
formed as a highly substructured, unbound association with a low volume density
(< 100 stars/pc^3). This is inconsistent with the idea that all stars form in
dense, compact clusters. The massive stars in Cyg OB2 show no evidence for
having formed particularly close to one another, nor in regions of higher than
average density. Since Cyg OB2 contains stars as massive as ~100 Mo this result
suggests that very massive stars can be born in relatively low-density
environments. This would imply that the massive stars in Cyg OB2 did not form
by competitive accretion, or by mergers.
