Time-varying surface temperatures of two flat-plate oscillating heat pipes (OHPs), one with Tesla valves and one without, were analyzed in the time–frequency domain using the short-time Fourier and Hilbert–Huang transforms. Tesla valves were installed along the channel structure of the OHP for the purpose of rectifying the oscillatory internal flow field, thereby enhancing heat transfer. Spatial-averaged surface temperatures in the evaporator region of both OHPs were investigated in the time–frequency domain at three separate heat inputs in order to detect salient effects of the Tesla valves. In all cases, the temperature signals from both OHPs were found to contain intermittent, aperiodic oscillations with most energy concentrated at frequencies in the 0–200 mHz range. The energies of oscillations in both OHPs were found to decrease with increases in the heat input, suggesting more consistent inter-channel flow circulation at these heat inputs heat inputs. The non-valved OHP temperature signals contained oscillations of larger amplitude and over a broader frequency range than the valved OHP temperature signals, indicating that the Tesla valves reduced the occurrence of intermittent high-energy oscillations in the OHP evaporator surface temperature. •The temperature signals for oscillating heat pipes (OHPs) are non-stationary.•OHP temperature oscillations occur predominantly at frequencies below 200 mHz.•‘Non-valved’ OHP temperature signals contain higher-energy oscillations.•Energy of OHP temperature oscillations decreases with increases in heat input.•‘Valved’ OHP temperature signals have less-deviant intrinsic mode functions.