Electron diffraction experiments and atomistic simulations have been performed on oxygen molecular clusters containing up to several hundreds Of 02 molecules, in order to determine their structural properties. Using the Etters-Kobashi-Belak potential [Phys. Rev. B 32 (1985) 4097] various low-energy stable conformations are constructed and compared after basin-hopping and parallel tempering Monte Carlo optimizations. Small clusters are generally seen to be icosahedral, and transform into cubic shapes above about 50 molecules. Precursors to the alpha crystalline phase are found to be even more stable for clusters containing as few as about 100 molecules. Simple models and finite temperature simulations allow as to construct a simplified size-temperature structural phase diagram for small (O-2)(n) clusters. (c) 2007 Elsevier B.V. All rights reserved.