We have measured the reaction rate coefficient and branching ratios for the H-3(+) + O atom reaction using a flowing afterglow/selected ion flow tube operating at room temperature (295 +/- 5 K). The measured rate coefficient was k = (1.2 +/- 0.5) x 10(-9) cm(3) s(-1) and the branching ratios were OH+ + H-2, (70%) and H2O+ + H (30%). This reaction has some relevance to the synthesis of water in interstellar clouds.