We present the results from a molecular beam study of the initial adsorption probability (S-0)of O-2 on Si(100)-2X1 as a function of surface temperature, incident kinetic energy and angle. The data show two distinct kinetic energy regimes with opposite temperature and energy dependencies, and correspond to two different adsorption mechanisms. For low incident kinetic energies, a trapping-mediated mechanism is dominant, exhibiting a strong increase in S-0 with decreasing surface temperature and kinetic energy. Also, adsorption at low kinetic energies is independent of incident angle, indicating total energy scaling. Data in this range are well-described by a simple precursor model, which gives a difference in activation barrier heights of (E-d-E-c)528 meV, and a ratio of preexponentials nu(d)/nu(c) =24.2. Trapping probabilities can also be estimated from the model, and show a strong falloff with increasing energy, as would be expected. At high incident kinetic energies, a strong increase in S-0 with kinetic energy indicates that a direct chemisorption mechanism is active, with the observed energy scaling proportional to cos theta(i). There is also an unusual increase in S-0 with surface temperature, with only a weak increase below 600 K, and a stronger increase above 600 K. The direct mechanism trends are discussed in terms of a possible molecular ion intermediate with thermally activated charge transfer. The molecular beam measurements are also used in calculating the reactivity of a thermalized gas with a clean surface. The precursor model is combined with a two-region fit of the direct adsorption data to predict chemisorption probabilities as a function of the incident conditions. These functions are then weighted by a Maxwell-Boltzmann distribution of incident angles and energies to calculate the adsorption probability for a thermal gas. These calculations indicate that the predominant mechanism depends strongly on temperature, with trapping-mediated chemisorption accounting for all of the adsorption at low temperatures, and direct adsorption slowly taking over at higher temperatures.