Silicon-oxycarbide optical waveguides and thermooptic devices include a substrate and a first cladding layer having a first refractive index positioned over a substrate. A first core layer comprising silicon, oxygen, and carbon and having a core refractive index is formed on the first cladding layer by chemical vapor deposition using at least two precursors: one inorganic silicon precursor gas and at least one second precursor gas comprising carbon and oxygen. Alternatively, at least three precursors can be used: one inorganic silicon precursor gas, a second precursor comprising carbon, and a third precursor comprising oxygen. The core layer is lithographically patterned to define waveguide elements. A second cladding layer having a second cladding refractive index is formed to surround the optical core waveguiding element of the first core layer.