Bulk Si(O)C ceramics are fabricated from polycarbosilane (PCS) by bulk pyrolysis along the route of cross linking, warm-pressing, and pyrolysis. The PCS was thermally oxidized for cross linking at various temperatures as a critical step toward the bulk transformation of PCS into bulk Si(O)C ceramics. The degree of cross linking of PSC affects the densities and bonding qualities of the warm-pressed powder compacts and, hence, the resultant ceramics. Under optimized processing conditions, crack-free bulk Si(O)C ceramics are obtained with a bulk density attaining 2.2 g/cm3. Despite the existence of a considerable amount of oxygen in the ceramics (16.08 wt%), resulting from the thermal oxidation processing, the ceramics show the characteristics of structures and properties of SiC-based ceramics. 29Si-solid state nuclear magnetic resonance spectra (NMR) reveal that the as-pyrolyzed X-ray amorphous Si(O)C phase consists mainly of SiC4 coordination units in the ceramic network, with the remainder being silicon-coordinated carbon and oxygen. Microhardness tests show that the as-pyrolyzed amorphous Si(O)C ceramics have a high hardness, attaining 24.91 GPa at a load of 2 N, and 19.82 GPa at a load of 10 N. Upon annealing at 1300°C in argon, the amorphous ceramics crystallized into nanophase β-SiC ceramics, and the ceramics kept the bulk nature of the amorphous ceramics with an increased density. The 29Si-solid state NMR spectrum indicates that the nanophase β-SiC ceramics consist of SiC4 units together with some mixed coordination units, namely SiO2C2 and SiOC3. The hardness of the crystallized nanophase Si(O)C ceramics attains 23.20 GPa at a load of 10 N. The present study demonstrates the possibility of fabricating bulk Si(O)C ceramics via the polymer-processing route, resulting in ceramics with promising structural and mechanical properties.