Abstract: Offshore wind turbine pile foundations are highly susceptible to significant local scour under combined wave–current action. However, most existing empirical formulas were developed from small-scale experiments or specific engineering scenarios, and their applicability to large-diameter offshore wind turbine monopiles has not yet been systematically assessed. To address this issue, this study reviews representative prediction formulas from domestic and international literature and expands the summary of their applicability ranges based on multiple engineering application studies, providing a reference for future formula selection in engineering prediction. In addition, the study uses available measured data to compare formulas developed for steady-current conditions with those for combined wave–current conditions. The results show that, under steady-current conditions, the selected formulas respond differently to variations in flow velocity and water depth. The Breusers and Melville formulas effectively capture changes in flow intensity, although their responses to water depth show a tendency toward saturation. The HEC-18 formula responds more moderately to both flow velocity and water depth. The S/M formula exhibits a pronounced nonlinear increase as the flow velocity approaches the threshold of sediment motion, whereas the modified 65-1 formula is controlled primarily by the flow-velocity term and shows little sensitivity to water depth. Under combined wave–current conditions, the predictions diverge significantly because of differences in the formulation of the wave–current coupling terms. Some formulas tend to underestimate scour depth under conditions of large pile diameter, coarse sand bed, and strong wave–current interaction. By contrast, formulas such as the Wang Rukai formula and the Han Haiqian formula with supplementary wave effects provide a relatively more complete representation of combined wave–current action, and the Wang Rukai formula generally gives more reasonable predictions for small- and medium-diameter pile foundations. Further analysis indicates that, under some offshore wind turbine conditions, dimensionless parameters such as pile diameter, sediment size, and KC number exceed the original applicability ranges of many empirical formulas, leading to trend deviations and failure of scale extrapolation. A comprehensive prediction strategy based on multi-formula comparison and regional correction using field measurements is therefore recommended to obtain more reliable design scour depths for offshore wind turbine pile foundations.