Single-crystal ZnO nanowires are synthesized using a vapor trapping chemical vapor deposition method and configured as field-effect transistors. Electrical transport studies show n-type semiconducting behavior with a carrier concentration of ∼107 cm-1 and an electron mobility of ∼17 cm2/V s. The contact Schottky barrier between the Au/Ni electrode and nanowire is determined from the temperature dependence of the conductance. Thermionic emission is found to dominate the transport mechanism. The effect of oxygen adsorption on electron transport through the nanowires is investigated. The sensitivity to oxygen is demonstrated to be higher with smaller radii nanowires. Moreover, the oxygen detection sensitivity can be modulated by the gate voltage. These results indicate that ZnO holds high potential for nanoscale sensing applications.