The OSS-BP-538 expansion backplane provides five PCIe 4.0 x16 downstream expansion slots for the latest PCIe add-in cards. Four downstream slots are dual width, allowing for dual width PCIe 4.0 x16 add-in cards such as GPUs to fit in one backplane. The OSS-BP-538 width is optimized to support two backplanes side-by-side in a standard 19” rackmount chassis. The expansion backplane connects back to the host node through the PCIe 4.0 x16 upstream slot which can be populated with an OSS-PCIe-HIB616-x16 PCIe 4.0 x16 host bus adapter. The OSS-BP-538 can also be bifurcated to support dual upstream host bus adapters for up to four downstream target add-in cards for double the total host-to-expansion throughput.