In this paper, we propose direct-detection optical orthogonal frequency division multiplexing superchannel (DDO-OFDM-S) and optical multiband receiving method (OMBR) to support a greater than 200 Gb/s data rate and longer distance for direct-detection systems. For the new OMBR, we discuss the optimum carrier-to-sideband power ratio (CSPR) in the cases of back-to-back and post transmission. We derive the analytical form for CSPR and theoretically verify it. A low overhead training method for estimating I/Q imbalance is also introduced in order to improve performance and maintain high system throughput. The experiment results show that these proposals enable an unprecedented data rate of 214 Gb/s (190 Gb/s without overhead) per wavelength over an unprecedented distance of 720 km SSMF in greater than 100 Gb/s DDO-OFDM systems.