We develop testbeds to study catalytic reactions modeled using the minimum number of platinum atoms needed to have an acceptable description of the chemistry on a realistic platinum surface that may include the contribution from a bulk continuum or may simply represent a local site on a nanocluster. In this particular case, the requirement is that a stable cluster may be connected to a stable bulk with equivalent highest occupied molecular orbital and Fermi level, respectively. We focus our work on the interaction of platinum clusters with molecular oxygen, which yields a complex cluster-molecule with fully delocalized molecular orbitals in the neighborhood of the Fermi energy of platinum, responsible for the interesting catalytic behavior of this material.