We have measured absolute cross section for the reaction of ground-state O+ with ammonia at collision energies in the range from near-thermal to approximately 15 eV, using the guided-ion beam (GIB) method. Measurements were also performed using ammonia-d(3) to aid in mass assignments. The reaction is dominated at low collision energies by charge transfer; however, the cross section for this exothermic channel is rather small, decreasing sharply with energy from similar to 40 angstrom(2) for normal ammonia at near-thermal energies and leveling off at 3.7 angstrom(2) above 6 eV; the cross section is slightly smaller for ammonia-d(3). Other channels, corresponding to the production of NH2+ and NO+, and possibly OH+, were detected. The NO+ channel, although nominally exothermic, is very small and exhibits a threshold at similar to 7 eV. Product recoil velocity distributions were also determined at selected collision energies, using GIB time-of-flight methods.