Widely applied optical oxygen sensors rely on quenching of the luminescence of an indicator by molecular oxygen. Although the state-of-the-art sensors are well suited for most applications and some are commercially available, there is still considerable interest for further improvements in applications like trace or ultra-trace sensing or measurements under harsh conditions (high pressures, temperatures, presence of organic solvents etc.). Conventional sensors are made in the form of planar foils or of fibre-optic setups and are commonly prepared by immobilization of molecular indicators into an oxygen-permeable matrix that acts as a support and solvent for the dye and a permeation-selective membrane. This is also true for nanoparticle-based oxygen probes used in biological and medical research, but indicators conjugated to proteins and peptides proved very successful alternatives. The performance of matrix-immobilized sensors is mainly determined by the properties of the indicator (spectral properties, photostability, luminescence decay time) and the matrix (chemical stability, oxygen permeability, permeation selectivity). Compatibility of both materials is important since aggregation of the indicator negatively affects the sensor performance. The sensitivity of optical oxygen sensors is mainly governed by two parameters: luminescence decay time of the indicator and oxygen permeability of the matrix. Considerably long decay times are required to achieve sufficient sensitivities in common matrices which explains why fluorescent dyes, popular in the early era of oxygen sensor development, are rarely used nowadays. Among numerous phosphorescent indicators explored metalloporphyrins are probably the most popular representatives. Palladium(II) and platinum(II) are almost exclusively used as central atoms; their porphyrin complexes possess phosphorescence lifetimes in order of tens and hundreds of microseconds, respectively. Immobilization of these indicators in polymers with moderate oxygen permeability such as polystyrene and its copolymers with acrylonitrile, poly(methyl methacrylate) etc. results in sensors operating at ambient conditions. On the other hand, highly gas-permeable matrices are required for preparation of sensors for traces of oxygen. These include polymers with high free volume such as silicone rubber, perfluorinated polymers of Teflon AF and Hyflon AD series and poly(1-trimethylsilyl-1-propyne), which is the polymer with the highest oxygen permeability ever reported. In a different approach, the indicators are physically adsorbed or covalently immobilized onto the surface of porous materials such as silica-gel or mesoporous silica where the dye is fully accessible to the interaction with molecular oxygen.