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Experimental demonstrations are reported for end-to-end real-time optical orthogonal frequency division multiplexing (OOFDM) transceivers incorporating three widely adopted adaptive loading techniques, namely, power loading (PL), bit loading (BL), and bit-and-power loading (BPL). In directly modulated distributed-feedback (DFB) laser-based, intensity-modulation, and direct-detection (IMDD) transmission systems consisting of up to 35-km single-mode fibers (SMFs), extensive experimental comparisons between these adaptive loading techniques are made in terms of maximum achievable signal bit rate, optical power budget, and digital signal processing (DSP) resource usage. It is shown that BPL is capable of supporting end-to-end real-time OOFDM transmission of 11.75 Gb/s over 25-km SMFs in the aforementioned systems at sampling speeds as low as 4 GS/s. In addition, experimental measurements also show that BPL (PL) offers the highest (lowest) signal bit rate, and their optical power budgets are similar. The observed signal bit rate difference between BPL and PL is almost independent of sampling speed and transmission distance. All the aforementioned key features agree very well with numerical simulations. On the other hand, BPL-consumed DSP resources are approximately three times higher than those required by PL. The results indicate that PL is a preferred choice for cost-effective OOFDM transceiver design.