SiBCN ceramics have excellent high-temperature stability and anti-oxidation properties, and they are candidates for materials used at extreme conditions. In this paper, amorphous SiBCN(O) ceramics were obtained through pyrolysis of cross-linked polyborosilazanes. The element content could be readily tuned through adjusting the structures of polymer precursors. The cerminization process and microstructure evolution process were systematically investigated. The SiBCN(O) ceramics remain amorphous below 1400 °C. The carbothermal reduction was suppressed because of the presence of boron. The SiBCN(O) ceramics showed 95.7% and 99.2% carbon residual in argon and air at 1600 °C, indicating high-temperature stability and good anti-oxidation properties. These SiBCN(O) precursors/ceramics could be potentially used in composites or protective coatings for harsh environments.