The overlapping frequency domain equalization (O-FDE) in digital signal processing (DSP) is frequently employed to provide dispersion compensation in long-distance coherent fiber optical communications. However, the change in overlapping symbol length that occurs during the processing of the O-FDE algorithm will typically be influenced by the decision and zero filling of the last subblock, which is harmful to the robustness of the O-FDE algorithm. In this study, with a thorough robustness analysis on changing overlapping symbol length, we present a novel method for de- cision and zero filling of the last subblock and examine the correspondingly resulting error vector magnitude (EVM) and symbol error ratio (SER) under different values of optical signal-to-noise ratio (OSNR), chromatic dispersion, and overlapped symbol lengths.