The reaction of O(P-3) + propyne (C3H4) was investigated at 298 K and 4 Torr using time-resolved multiplexed photoionization mass spectrometry and a synchrotron-generated tunable vacuum ultraviolet light source. The time-resolved mass spectra of the observed products suggest five major channels under our conditions: C2H3 + HCO, CH3 + HCCO, H + CH3CCO, C2H4 + CO, and C2H2 + H-2 + CO. The relative branching ratios for these channels were found to be 1.00, (0.35 +/- 0.11), (0.18 +/- 0.10), (0.73 +/- 0.27), and (1.31 +/- 0.62). In addition, we observed signals consistent with minor production of C3H3 + OH and H-2 + CH2CCO, although we cannot conclusively assign them as direct product channels from O(P-3) + propyne. The direct abstraction mechanism plays only a minor role (<= 1%), and we estimate that O(P-3) addition to the central carbon of propyne accounts for 10% of products, with addition to the terminal carbon accounting for the remaining 89%. The isotopologues observed in experiments using d(1)-propyne (CH3CCD) and analysis of product branching in light of previously computed stationary points on the singlet and triplet potential energy surfaces (PESs) relevant to O(P-3) + propyne suggest that, under our conditions, (84 +/- 14)% of the observed product channels from O(P-3) + propyne result from intersystem crossing from the initial triplet PES to the lower-lying singlet PES.