Oxide dispersion-strengthened (ODS) steels are attractive materials for use in a wide variety of high-temperature power production applications including fusion and fission reactors, space energy applications, and conventional power plant components. Their high-temperature strength and radiation damage resistance is a result of nanoscale dispersed oxides, which pin grain boundaries and dislocations while also providing sinks for hydrogen and helium atoms, mitigating radiation-induced swelling. Traditional fusion joining processes are unsuitable for these alloys, because the oxide particles agglomerate during melting. Friction stir welding (FSW) is a solid-state joining process thought to prevent these effects, but we present experimental evidence of yttrium oxide particle agglomeration, coarsening, and transformation after FSW of a MA956 ODS steel. We discuss both the base metal and stir zone microstructures, as studied by scanning electron microscopy, energy dispersive X-ray spectroscopy, and atom probe tomography.