The invention relates to an optical sensor element, comprising indicators, selected from luminescence-active means that are of the same type or different, and indicator protectors, and to a sensor, comprising at least one such sensor element, an energy source that excites the luminescence emission of the indicators, and a detector unit, wherein the sensor element or sensor is suitable for detecting molecular oxygen in a gaseous or liquid medium and/or for determining the molecular oxygen content of a gaseous or liquid medium and at least one layer of the sensor element bearing the indicator protectors is designed in such a way that the diffusion rate of the molecular oxygen formed on the indicator protectors by means of the reduction of strong oxidants back into the medium is greater than the diffusion rate of molecular oxygen from the medium in the direction of the at-least-one layer bearing the indicator molecules.