Point-to-multipoint （P2MP） optical transceivers enable multiple low-speed optical transceivers at different locations to be able to simultaneously communicate with a few high-speed optical transceivers concentrated or distributed in network central offices，thus offering a cost-effective future-proof solution for 5G and beyond. A novel intensity modulation/ direct （IMDD） P2MP optical transceivers incorporating inverse fast Fourier transform （IFFT）/ fast Fourier transform （FFT）-based multi-channel aggregation/de-aggregation has been thoroughly explored the scalability and further evaluated its potential in upstream IMDD PONs. It cannot only operate at the‘add-as-you-grow’mode，but also offer additional physical layer network security. The results show that the P2MP flexible optical transceivers equip the PONs with excellent scalability. Each ONU can adaptively change its channel count without considerably compromising transmission capacity and ONU launch power dynamic range. The increase in ONU count can also be made available by slightly compromising transmission capacity and ONU launch power dynamic range. Therefore，the flexible P2MP optical transceiver can effectively improve the scalability in IMDD PONs，and provide a technical solution for building a flexible，elastic and scalable communication network in the future.