We present a robust frequency synchronization algorithm for the Coherent optical Offset quadrature-amplitude modulation based Filter-bank multicarrier (CO-FBMC-OQAM) systems. It is a two-stage method operating in the frequency domain, which expands the estimated range of the normalized fractional frequency offset to (-1;+1). We designed and introduced a cascading cross-correlation function of real-valued pilots to roughly estimate the frequency offset in the first stage. The residual frequency offset was accurately tracked by a non-cascading cross-correlation function afterwards. This approach enlarges the maximum estimated range, and also resists interference and noise. A CO-FBMC-OQAM system with the sampling rate of 50 Gsample/s was numerically investigated to evaluate the frequency-offset estimation and compensation capability. All obtained results prove that the proposed frequency synchronization method can correct the laser-frequency offset and improve the transmission performance significantly. © 2020 Chinese Institute of Electronics.