A dissolved oxygen sensor based on fluorescence quenching of the oxygen-sensitive ruthenium complex, [Ru(II)-tris(4,7-diphenyl-1,10-phenanthroline]2+, which has been immobilised in a porous silica sol-gel-derived film, is reported. Ormosil sensing films are fabricated using the modified silica precursors methyltriethoxysilane (MTEOS) and ethyltriethoxysilane (ETEOS), and are dip-coated onto planar glass substrates. Due to their increased hydrophobicity compared with conventional sol-gel precursors, such as tetraethoxysilane (TEOS), the use of these modified precursors enables the fabrication of the optimum sensor films for the purposes of dissolved oxygen sensing. This project aims to investigate a number of issues that have been identified as being of crucial importance for optimum sensor performance. In particular, the long-term stability of the sensor films is investigated and issues such as dye leaching and photobleaching are discussed. The development and optimisation of a suitable barrier layer for the purpose of optical isolation of the sensor films is also detailed. Additionally, an investigation to establish the temperature dependence of the sensor is reported. Finally, the progress towards a commercial instrument is documented.