Supersonic molecular beam techniques were used to determine the initial dissociative chemisorption probability, S-0, of O-2 on the Ru(001) surface as a function of incident kinetic energy, E(i), angle of incidence, theta(i), and surface temperature, T-s. For T-s between 77 and 900 K, two kinetic energy regimes are observed as E(i) increases from 30 to 700 meV. In this article we discuss the mechanism for chemisorption in the low kinetic energy regime. S-0 decreases with E(i) for incident energies less than similar to 70 meV, and there is little dependence of S-0 on theta(i), in this low energy range. Additionally, S-0 decreases rapidly as T-s is increased in this regime, and we believe that a simple trapping-mediated mechanism dominates adsorption. The low E(i) data are fit to a simple one-dimensional potential energy surface model that includes the molecule initially trapping into a physical adsorption well and then surmounting a barrier into the dissociatively chemisorbed state. Analysis of an Arrhenius construction yields a difference in activation energies for desorption and dissociation from the physically adsorbed state that is approximately 28 meV below the vacuum zero, for both E(i) = 30 and 53 meV.