A dissolved oxygen sensor, based on sol-gel-derived silica thin films impregnated with an oxygen-sensitive ruthenium complex, is reported. Porous sol-gel silica films, dipcoated onto either planar glass substrates or declad optical fibers, are doped with the complex [RuII-tris(4,7-diphenyl-1,10-phenanthroline)], whose fluorescence emission is quenched by oxygen. The complex is entrapped in the cage-like structure of the sol-gel matrix, but is accessible to oxygen via the microporous channels. This work compares the difference in oxygen quenching response between gas phase and aqueous phase measurements. Optimization of dissolved oxygen response by tailoring of the film fabrication parameters is reported. Using a high-brightness blue LED, combined with a miniature photodiode-based detection system, these results establish the viability of a low-cost, high-performance, portable optical dissolved oxygen sensor.