Oops
From BeatlesWiki
OOPS means Out Of Phase Stereo. It is an effect obtained by combining the sound of one stereo channel with an inverted image of the other channel. It is also called "Left Minus Right" or simply "L-R" What is the result? First, we must remember that, if a wave is combined with its inverted image, they will cancel themselves and the result will be no oscillation - if we are dealing with sound waves, the result will be silence. This way, it's easy to understand that, by combining the sound of one channel with the other channel inverted, everything that is EQUAL on both channels will DISAPPEAR, and the remaining sound will be a mono waveform combining everything that is "different" between the channels - usually sounding very different than the original recording.
