The present application describes an optical sensor for measuring oxygen gas levels in a medium. The optical sensor includes a substrate having a first and second surface. The optical sensor also includes a first coating applied on the first surface of the substrate. The first coating may include an oxygen impermeable matrix doped with a first fluorophore. The optical sensor may include a second coating applied on the substrate. The present application also describes a capnography system for measuring oxygen including an optical sensor and an algorithm to estimate the maxima of oxygen levels from instantaneous oxygen levels and calculating instantaneous carbon dioxide levels from the difference between average maximum oxygen gas level and instantaneous oxygen gas level.