A free‐fall ocean‐bottom seismograph (OBS) cannot always achieve the desired seismometer leveling because of the impact of the descending OBS with the seafloor and the complexities of the ocean bottom. How a failure of the leveling system of OBS affects seismic applications, especially those relying on three‐component seismograms such as the receiver function (RF), needs to be carefully explored. Here, we investigate the effects of a tilted seismometer on the RF analysis using data from a passive‐source OBS experiment in the South China Sea. The back azimuth–dependent, negative polarity of P signature observed on RF traces of two OBSs can only be explained by tilted seismometers. Analyses of tilt noise independently confirm that seismometers in these two OBSs were permanently tilted. By correcting the tilt, we are able to greatly enhance the signals of the Moho‐associated P‐to‐S‐converted phases on the RFs.