Herein, we report a two‐step process forming arene C−O bonds in excellent site‐selectivity at a late‐stage. The C−O bond formation is achieved by selective introduction of a thianthrenium group, which is then converted into C−O bonds using photoredox chemistry. Electron‐rich, ‐poor and ‐neutral arenes as well as complex drug‐like small molecules are successfully transformed into both phenols and various ethers. The sequence differs conceptually from all previous arene oxygenation reactions in that oxygen functionality can be incorporated into complex small molecules at a late stage site‐selectively, which has not been shown via aryl halides. Efficient diversification: The first site‐selective late‐stage aromatic C−O bond formation from arene C−H bonds proceeds in two steps via arylthianthrenium intermediates. Electron‐rich, ‐poor and ‐neutral arenes as well as complex drug‐like small molecules are successfully transformed into both phenols and various ethers.