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Photoionization of oxygen molecules by high-order harmonics was investigated. High-order harmonics of a Ti:sapphire laser produced in a Kr gas cell were used to excite autoionization states of O2. Since the high-order harmonic source used contains several harmonic orders, the resulting photoelectron spectrum also showed multiple peaks coming from different orders of harmonics. A subtraction method using known photoionization cross-sections was employed to separate out individual contributions from the harmonics. The photoelectron spectrum from the 11th harmonic shows a clean contribution from an autoionizing state.{