OC should save power on high-load time-independent processes. If it can complete the process in a shorter time, then it can switch back to a lower power mode quicker. Playing a video is time-dependent though, so unless it can switch down for micro-seconds between frames (which may depend on the CPU governor) then it'll be running at full power for the entire time. Something like a compression job should allow the CPU to complete the task faster and switch back to low power mode quicker (providing it's not I/O bound anyway).