A simple, digital signal processing-free, low-cost and robust synchronous clocking technique is proposed and experimentally demonstrated, for the first time, in a 64-QAM-encoded, 11.25Gb/s over 25km SSMF, real-time end-to-end optical OFDM (OOFDM) system using directly modulated DFB laser-based intensity-modulation and direct-detection (IMDD). Detailed experimental investigations show that, in comparison with the common clock approach utilised in previous experimental demonstrations, the proposed clocking technique can be implemented to achieve no system BER performance degradation or optical power budget penalty and more importantly to improve system stability. As a viable synchronous clocking solution for real-time OOFDM transmission, this work is a vital step towards the realisation of practical OOFDM transmission systems and has particular significance for synchronisation of OOFDM multiple access-based passive optical networks where highly accurate synchronisation of all network elements is essential.