Reduced forms of iron, such as zero-valent ion (ZVI) and ferrous ion (Fe[II]), can activate dissolved oxygen in water into reactive oxidants capable of oxidative water treatment. The corrosion of ZVI (or the oxidation of (Fe[II]) forms a hydrogen peroxide (H₂O₂) intermediate and the subsequent Fenton reaction generates reactive oxidants such as hydroxyl radical (^●OH) and ferryl ion (Fe[IV]). However, the production of reactive oxidants is limited by multiple factors that restrict the electron transfer from iron to oxygen or that lead the reaction of H₂O₂ to undesired pathways. Several efforts have been made to enhance the production of reactive oxidants by iron-induced oxygen activation, such as the use of iron-chelating agents, electron-shuttles, and surface modification on ZVI. This article reviews the chemistry of oxygen activation by ZVI and Fe(II) and its application in oxidative degradation of organic contaminants. Also discussed are the issues which require further investigation to better understand the chemistry and develop practical environmental technologies.