Proper orthogonal decomposition was employed to turbulent flows over an open cavity to investigate the spatial characteristics of coherent structures responsible for self-sustained oscillations. The distributions of pressure fluctuations were analyzed by using snapshot-based proper orthogonal decomposition. When self-sustained oscillations took place for Re-D = 12, 000 in the cavity geometry of L/D = 2, three pairs of alternative patterns were shown in the first and second modes of pressure fluctuations. By examining both temporal evolutions and spanwise distributions of the first two modes, quasi-two-dimensional vortical formations were shown to be responsible for the self-sustained oscillations. In turbulent cavity flows of Re-D = 3000, however, the periodic oscillations were not observed, due to the irregular shedding of three-dimensional vortical structures. For the application of proper orthogonal decomposition analysis to experimental data, the decomposition was employed to the spatial distributions of v-v correlations on the lip line of cavity geometry.