The geologic origin of the ancient, phyllosilicate-bearing bedrock at Oxia Planum, Mars, the ExoMars rover landing site, is unknown. The phyllosilicates record ancient aqueous processes, but the processes that formed the host bedrock remain elusive. Here, we use high-resolution orbital and topographic datasets from the HiRISE, CaSSIS and CTX instruments to investigate and characterize fluvial sinuous ridges (FSRs), found across the Oxia Planum region. The FSRs form segments up to 70 km long, are 20-600 m wide, and up to 9 m in height, with sub-horizontal layering common in ridge margins. Some FSRs comprise multi-story ridge systems; many are embedded within the phyllosilicate-bearing bedrock. We interpret the FSRs at Oxia Planum as deposits of ancient, episodically active, alluvial river systems (channel-belt and overbank deposits). Thus, at least some of the phyllosilicate-bearing bedrock was formed by ancient alluvial rivers, active across the wider region, though we do not exclude other processes from contributing to its formation as well. The presence of alluvial floodplains at Oxia Planum increases the chances of the ExoMars rover detecting signs of ancient life. Future exploration by the ExoMars rover can verify the alluvial interpretation and provides an opportunity to investigate some of the oldest river deposits in the Solar System.