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This paper investigates the performance of some recently proposed optical orthogonal frequency-division multiplexing (OFDM) transmission techniques in a short-range intensity-modulated, direct-detected (IM/DD) optical channel. Due to the unipolarity of the IM/DD channel, asymmetric clipping and associated nonlinear distortion is inevitable, resulting in an unfavorable dc power and a significant performance penalty. Using a detailed analysis of the clipped signal, our recent work focused on reducing this clipping-resulted dc via both power optimization and frequency planning. In this paper, a set of effective receiver signal-to-noise ratio formulas is proposed, which helps in accurately and efficiently evaluating the system performance across various optical OFDM systems. We conduct a power efficiency analysis to reveal tradeoffs between power and spectral efficiencies among different systems. Our results show that with a cost of additional complexity, a diversity-combining technique offers significant benefits in all cases.