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We develop a theoretical expression to predict the ultimate back-to-back performance of coherent optical orthogonal frequency-division multiplexing (CO-OFDM) systems that rely on four-wave mixing to achieve phase conjugation for mid-span spectral inversion. Our analysis shows that two different two-stage nonlinear processes produce strong noise-like products in the conjugated signal band. We verify our theoretical results with simulations and experiments; these both show excellent agreement with the analytical theory. We identify the optimal design parameters and predict that optical phase conjugation of 10 THz wide orthogonal frequency-division multiplexing signals could be possible, given appropriate dispersion management of the nonlinear element. We also experimentally demonstrate the benefit of MSSI in an 800 km transmission of CO-OFDM.