The evolution of the atmosphere of Mars and the loss of volatiles over the lifetime of the solar system has been a key motivation for the Mars Atmosphere and Volatile Evolution (MAVEN) mission. Studies based on MAVEN data have demonstrated that the major atmospheric oxygen loss process is photochemical — that is, the escape of fast O atoms produced by dissociative of the major ionospheric ion species, O2+. A hot oxygen corona is also produced by this process and some O loss is due to ion loss associated with the solar wind interaction with the planet. Note that at Venus and Earth, all the hot O atoms produced by the dissociative recombination reaction have speeds less than the escape speed and cannot directly escape, although ion loss is possible. Using our Mars-based knowledge, this talk will consider how we can extend our understanding of this key atmospheric loss process to different size Mars-like exoplanets and to different levels of stellar ionizing radiation. The effects of different upper atmosphere compositions will also be discussed. Oxygen loss has direct relevance to the retention of volatiles (whether carbon dioxide or water) by planets, and thus to issue of habitability.