Fiber nonlinear effects are found to cause severe signal degradation in long-haul coherent optical OFDM (CO-OFDM) transmission, particularly in dispersion-managed transmission. We present promising techniques to mitigate the fiber nonlinear impairments in both single-channel and wavelength division multiplexed (WDM) transmission. For single-channel transmission, we discuss a joint self-phase modulation (SPM) compensation scheme that is applicable to polarization division multiplexed (PDM) CO-OFDM signals. Experimental demonstration of this joint SPM compensation scheme is also presented. For WDM transmission, we describe an OFDM frame design and a dispersion management scheme that reduce the cross-phase-modulation (XPM) penalty among the WDM channels. The impact of XPM from WDM channels with different signal formats and data rates is briefly discussed. é 2010 Alcatel-Lucent.