The detection of oxygen is important and optical sensors for this purpose are of increasing interest, especially in modified atmosphere food packaging (MAP), in which the food package is flushed with a gas, such as carbon dioxide or nitrogen. An oxygen optical sensor for MAP should be inexpensive, rapidly-responding and irreversible. The reasons behind the need to develop such a sensor for MAP are discussed. The different types of indicator that have been developed to date fall mainly into the following categories: reversible luminescent indicators, reversible colourimetric indicators and visible and/or UV light activated indicators. The basic technologies underpinning each of these three different indicator types are described, their various typical components detailed and examples of the indicators in action given. The potentials of these different indicators for application in MAP are evaluated. Particular attention is given to a new irreversible, reusable, colourimetric, UV-activated oxygen indicator that shows great potential.