An iterative digital signal processing (DSP) for reducing signal-to-signal beating interference (SSBI) in a multiband direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) system is proposed and demonstrated experimentally. Without applying any optical/electrical filter in the receiver, all the OFDM signal bands can be almost uniformly recovered even under severely SSBI contamination. The aggregated data rate in the experimental demonstration achieves 65.4 Gb/s by employing 16-QAM format. Meanwhile, by applying a simulation aided investigation, the spectral efficiency of the proposed 16-QAM DDO-OFDM signals are enhanced to 3.4 b/s/Hz. The results indicate not only high spectral efficiency and high uniformity performance among all OFDM signal bands under different level of SSBI pollution, but also high potential of a multiband DDO-OFDM system to be widely employed.