In this work the resonance hairpin probe technique has been used for detection of photoelectrons generated during photodetachment experiments performed to determine negative ion density in an inductively coupled oxygen plasma. An investigation of the temporal development of the photoelectron population was recorded with the hairpin probe located inside the laser beam region and at various points outside the beam. Varying the external microwave frequency used to drive the probe resonator allowed the local increase in electron density resulting from photoelectrons to be determined. At a fixed probe frequency, we observed two resonance peaks in the photodetachment signal as the photoelectron density evolved as a function of time. Inside the laser beam the resonance peaks were asymmetric, the first peak rising sharply as compared with the second peak. Outside the laser beam region the peaks were symmetric. As the external frequency was tuned the resonance peaks merge at the maximum electron density. The resonance peak corresponding to maximum density outside the beam occurs at a delay of typically 1–2 µs as compared with the centre of the beam allowing an estimate of the negative ion velocity. Using this method, negative ion densities were measured under a range of operating conditions inside and outside the beam.