This paper provides an analysis of the performance of conventional preamble-based time synchronization techniques, which have been proposed for Orthogonal Frequency Division Multiplexing (OFDM) radio transmissions, in the context of optical Intensity Modulated and Direct Detected (IMDD) OFDM transmissions over single-mode fiber. A novel preamble structure along with a two-stage synchronization algorithm is proposed. Thanks to the particular structure of the preamble, a first coarse synchronization metric does not present minor peaks. In addition, without the knowledge of the preamble waveform the receiver can perform both coarse and fine synchronization steps. As maximal delay in arrival time between different frequency components of a signal depends on chromatic dispersion and the signal bandwidth, it is also observed that a slowly time-varying preamble is more robust than a fast time-varying one. By means of numerical simulations, the proposed method is shown to provide better performance when compared to other conventional methods in terms of timing offset variance with reduced receiver complexity. In unamplified transmissions the proposed technique guarantees a quasi-optimal transmission capacity even with a short cyclic prefix.