The deuterium isotope effect on product energy partitioning in the O- + H-2 particle transfer reaction is investigated in a crossed molecular beam experiment. Vibrational-state-resolved angular distributions are measured at six collision energies between 0.20 and 0.77 eV for the O- + H-2 reaction and at seven collision energies between 0.22 and 1.20 eV for the O- + D-2 reaction. The fraction of the total available energy deposited into product vibration is significantly larger in the deuterium system than in the hydrogen system. This effect is greatest at the lowest collision energies where OD- products are formed with more than twice as much vibrational energy as OH-products. The isotopic systems display similar trends in the product angular distributions, which extend over the full range of scattering angles at low energies and shift towards the forward direction as the collision energy is increased. These observations are discussed in terms of a competition between reaction mechanisms. An insertion-migration mechanism, yielding products with moderate vibrational excitation, is especially important at the lower energies. The insertion process leads to the isotope effect in the product energy partitioning which is explained in terms of Franck-Condon factors. As the energy increases, larger impact parameter collisions are able to proceed through a direct mechanism, yielding more tightly forward-scattered, vibrationally excited products. Since direct mechanisms show isotopically independent energy partitioning, the overall isotope effect diminishes with increasing energy as more collisions become purely direct. Bimodal rotational state distributions help strengthen the claim that two distinct reaction mechanisms produce the particle transfer product.