Digital filter multiple access (DFMA) passive optical networks (PONs) exploit centralized software defined networking (SDN) controller-managed and transceiver-embedded digital orthogonal filters in individual optical network units (ONUs) to enable end-users to adaptively and dynamically access and share a common fiber transmission medium. DFMA PONs have the potential of not only equipping NG-PONs with sufficient network operation flexibility, adaptability, elasticity and reconfigurability, but also providing highly desirable backwards compatibility with current PON standards. In this paper, for the first time, multipoint-to-point upstream signal transmission in intensity-modulation and direct-detection (IMDD) DFMA PONs is experimentally demonstrated using two real-time, reconfigurable, optical orthogonal frequency division multiplexing (OOFDM)-modulated ONUs and an offline optical line terminal (OLT). Experimental demonstrations show that each ONU achieves similar upstream bit error rate (BER) performance, excellent tolerance to inter-ONU sample timing offset and a relatively large ONU launch power variation range.