This paper is concerned with sliding mode control of offshore steel jacket platforms subject to nonlinear wave-induced forces. Two sliding mode control schemes are proposed to reduce the oscillation amplitudes of the platforms. One scheme is that a conventional sliding mode controller (SMC) is designed, under the SMC the oscillation amplitudes of the platform are effectively reduced; however, a very large control force is required. The other scheme is that by intentionally introducing a time-delay into the control channel, a delayed sliding mode controller (DSMC) is designed. Compared with the SMC, the DSMC improves the control performances in two aspects: (i) the oscillation amplitudes of the first, second and third floors under the control of the DSMC are less than those under the control of the SMC; (ii) the control force required by the DSMC is much less than that by the SMC, which are confirmed by simulation results.