Polycarbosilane-based thermosets exhibit promising low-dielectric properties but relatively low thermal resistance. In this work, to overcome this drawback, siloxane units were incorporated into the main chain of polycarbosilane via hydrolytic condensation to generate oligomers with siloxane–carbosilane hybridized main chains (oligomeric PCOSs). These oligomers enabled subsequent ultraviolet curing via the reaction of disilacyclobutene and thermal curing via the reaction of benzocyclobutene on the pendant groups to form cured resins. The cured resins were photopatternable, and not only showed enhanced thermal resistance compared with that of polycarbosilane-based thermosets, but also maintained outstanding low-dielectric properties.