The ion energy distribution in oxygen plasma in a planar inductive discharge was measured in the pressure range 3-20 mTorr using a quadrupole mass spectrometer. The ion energy distribution is broad and bimodal at low pressures, indicating that there is capacitive coupling between the rf coil and the discharge. At higher pressures multiple peaks generally related to charge-exchange processes in the sheath are apparent. The mean ion energy increases with decreasing gas pressure, from about 10 eV at 20 mTorr to roughly 25 eV at 3 mTorr. The mean ion energy follows closely the d.c. plasma potential measured by a Langmuir probe. The mean ion energy of O+ ions is generally 10-40% higher than the mean ion energy of O2+ ions.