Abstract: The basic principles of oxygen optical sensors are briefly described. A general overview of the influence of the fluorophore and, mainly, of the immobilizing matrix on sensor performance is made. The sol-gel preparation and the properties of the sensor based on Ru(II) tris(4,7-diphenyl)-1,10-phenanthroline entrapped in silica matrix are discussed. The influence of Si-precursor nature (and especially the effect of the organically modified silanes) and films deposition mode (dip- or spin-coating) on the morphology, sensitivity and linearity of the Stern-Volmer dependence are outlined. Negative consequences of the Ru-complex microcrystallization in the matrix on sensor performance are confirmed. By avoiding such crystallization through sonication of the sol, proposed by the authors, the sensor properties are significantly improved and Stern-Volmer constant values are increased by 5−10 times compared with reported literature data. The films produced in different batches show good reproducibility of the analytical results and are successfully tested for oxygen determination in the gaseous phase, natural waters (lakes of Smolyan), beer, and wine. Self-photoluminescence of the latter and carbon dioxide present do not interfere significantly with the results from analysis.