This paper proposes a simple and efficient approach for the estimation of carrier frequency offset (CFO) in coherent optical OFDM (CO-OFDM) systems. By zero-padding the first received OFDM symbol and performing discrete Fourier transform (DFT), the obtained spectrum will preserve the shape of the original transmitted spectrum, but with finer frequency intervals. Leveraging this feature and the matched filter concept, we can develop a CFO estimator that can achieve the optimum estimation performance whose estimation error variance can reach the Cramer-Rao lower bound (CRLB). However, to obtain the accurate the estimation of CFO, the zero-padding rate should be large enough and a search step is required in matched filtering. In order to reduce the implementation complexity of the proposed algorithm, we further simplify the cost function after matched filtering to an approximated cosine function. Given three test values, the CFO estimate can be directly calculated from the test functions, without complicated search. Simulation results verify the feasibility and simplicity of the proposed simplified CFO estimator.