Sampling frequency offset (SFO) will result in phase rotation, inter-channel interference, and inter-symbol interference, which can seriously degrade the transmission performance of the asynchronous direct-detection optical OFDM (DDO-OFDM). In this paper, we study a simple and overhead-free SFO blind estimation technique in a DDO-OFDM system. The key parameters in this technique are optimized by theoretical analysis, numerical simulation, and experiments. A simplified method of the SFO blind estimation is also proposed to further reduce the hardware implementation complexity. The accuracy and stability of the SFO estimation are experimentally investigated in the real-time asynchronous DDO-OFDM transmission system. The results indicate that the SFO estimation accuracy can be within ±1.5 ppm and shows high stability even at an SFO of up to 200 ppm after 20-km standard single-mode fiber transmission.