In this paper, we study the impact of phase noise (PN) in direct- detection optical orthogonal frequency division multiplexing (DDO-OFDM) systems. We derive an approximate closed-form expression of bit error rate (BER) estimation for subcarrier channels. Moreover, by investigating the phase noise properties, we put forward a relatively precise algorithm for BER performance in 16QAM-DDO-OFDM and 64QAM-DDO- OFDM systems. We also demonstrate the algorithm precision through comparison between the theoretical derivation and the simulation results.