Guided-ion beam cross section and product ion time-of-flight (TOF) measurements are presented for the O+(S-4) + C2H2 reaction over the center-of-mass collision energy range of 0.05-18 eV. Despite a large number of exothermic channels, the total reaction cross section at low energies is more than two orders of magnitude smaller than the capture cross section. A common energy onset for charge-transfer, CH+, and COH+/HCO+ products is observed at 1.70+/-0.10 eV, above which the total cross section for these channels rapidly rises with energy, eventually exceeding 5 Angstrom(2). Above 4 eV, the C2H+ and CO+ products also become significant, and weaker channels producing C+, C-2(+), and CH2O+ are also identified. The C2H+ fragment is interpreted as being primarily a dissociative charge transfer (CT) channel at collision energies above similar to 3.79 eV, while the threshold for forming CO+ can be associated with a CO+ + H + CH channel. The TOF measurements demonstrate that efficient C2H2+ (A) over tilde state formation occurs above similar to 2.7 eV. The onset at 1.70+/-0.10 eV is attributed to a transition state associated with an excited quartet hypersurface.