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Although time-domain oversampling of the received baseband signal is common for single-carrier transmissions, the counterpart of frequency-domain oversampling is rarely used for multicarrier transmissions. This is because frequency-domain oversampling cannot be taken advantage of, when using the commonly used low-complexity receiver that assumes orthogonal subcarriers. In this paper, we explore frequency-domain over-sampling to improve the system performance of zero-padded (ZP) orthogonal frequency division multiplexing (OFDM) transmissions over underwater acoustic channels with large Doppler spread. In these channels, intercarrier interference (ICI) has to be addressed explicitly via frequency-domain equalization, which enables inclusion of additional frequency samples at little increased complexity. We use a signal design that enables separate sparse channel estimation and data detection, reducing equalization complexity. Based on both simulation and experimental results, we observe that the receiver with frequency-domain oversampling outperforms the conventional one considerably, where the gain increases as the Doppler spread increases.