The reactions of O(P-3) with CH4, CH3CH3, and CH3CH2CH3 at center-of-mass collision energies in the range of 2.8-3.9 eV have been investigated with crossed-beams experiments and with direct dynamics calculations. The experiments and calculations both provide evidence for previously unobserved reaction pathways which principally lead to O-atom addition and subsequent H-atom elimination or C-C bond breakage: O(P-3) + RH --> RO + H or R'O + R". In addition, the expected H-atom abstraction reaction to form OH has been observed. The H-atom abstraction reactions have modest barriers in the range similar to0.1-0.3 eV, whereas the addition pathways have barriers greater than similar to1.8 eV. Nevertheless, theory predicts that abstraction and addition reactions occur with similar probabilities at the collision energies of these studies. Although high barriers prevent the addition reactions from occurring in most thermal environments, such reactions might be important in low-Earth orbit, where spacecraft surfaces and exhaust gases suffer high-energy collisions with ambient atomic oxygen.