We analyze the intercarrier interference (ICI) induced by residual frequency offset (RFO) in optical fast orthogonal frequency division multiplexing (F-OFDM) and compare this scheme with conventional OFDM in two scenarios: (1) the same RFO to subcarrier spacing ratio, and (2) the same discrete cosine or Fourier transform point size. In both scenarios, the ICI on a subcarrier in F-OFDM is dominated by the adjacent subcarriers, but the contribution to that in conventional OFDM is dispersed across the subcarriers. 19.5-GBaud 16QAM and 4ASK F-OFDM systems are investigated. 16QAM F-OFDM exhibits an RFO tolerance up to six times greater than that of 16QAM OFDM, when equalizers with the same tap number are used for the RFO compensation. 4ASK F-OFDM using 1-tap equalizers can achieve an RFO tolerance of up to four times greater than that of five-tap equalizer-based 4ASK OFDM.