We propose a wavelength-reuse coherent orthogonal frequency division multiplexing (OFDM) uplink optical transmission scheme that employs downlink intensity modulation (IM) and uplink phase modulation (PM). In the proposed approach, the uplink OFDM signal is generated by a single optical phase modulator, which uses the intensity-modulated downlink signal as its optical source. Consequently, the uplink signal contains both IM and PM components. By leveraging the orthogonality between IM and PM in coherent detection, the PM component—which represents the desired uplink signal—can be effectively recovered. Since optical phase modulation provides a larger Euclidean distance than optical intensity modulation, higher signal power can be reliably achieved through coherent detection. Furthermore, the proposed uplink transmitter eliminates the need for an optical source as well as temperature or bias controllers, thereby simplifying the overall transmission scheme. We experimentally demonstrated bidirectional 10-km optical transmission using two 1-GHz 16QAM-OFDM signals centered at 1.9 and 3.4 GHz, successfully validating the proposed technique.