The impacts of the channel frequency response roll-off effect on upstream optical OFDM transmission performance in digital filter multiple access (DFMA) passive optical networks (PONs) based on intensity modulation and direct detection (IMDD) are investigated, in terms of BER versus received optical power performance, ONU transmission capacity, ONU launch power variation (LPV) range and inter-ONU sample timing offset (STO). The effect of signal bandwidth is also explored by comparing tolerance to the channel roll-off effect for two different signal bandwidths. Over all of the aforementioned aspects, good agreements are obtained between numerical simulations and experimental measurements. The results provide important insights into channel roll-off-induced performance dependencies to facilitate cost-effective designs of both DFMA transceivers and IMDD DFMA PONs.