The unsteady behaviors of adiabatic effectiveness under oscillating flow conditions were studied extensively using the fast-response pressure-sensitive paint technique. Nitrogen was used as the coolant and was discharged from a 35°-inclined round hole. The oscillation frequencies of the mainstream were set to 0, 5, 10, and 20 Hz. The unsteady features of the measured effectiveness were quantified in terms of standard deviations, cross correlations, proper orthogonal decomposition (POD), and dynamic mode decomposition (DMD). Compared to the steady state, the measured effectiveness for the f = 10 and 20 Hz cases varied greatly during each cycle and led to significant enhancement in fluctuations. Cross correlations were used to determine the spatial signatures of large-scale structures (i.e., counterrotating vortex pairs, the horseshoe vortex, and structures near the hole) at various oscillation frequencies. Further POD and DMD analyses successfully decomposed the instantaneous fluctuations into a series of energetic modes, which vividly illustrated the strong modulations in adiabatic effectiveness imposed by flow oscillations and unsteady events.