The iodine-catalyzed catalytic formal [3 + 3 + 1] cycloaddition for the preparation of a seven-membered O-heterocyclic ring is presented, which is an achievement of methyl and carbonyl group reactivity of 3-methyl-5-pyrazolones to forge the Csp3-O bond. This novel protocol provides a straightforward and efficient access to structurally diverse fused O-heterocycles through an iodine-catalyzed iodination/Kornblum oxidation/oxidative coupling/C-O bond formation cascade reaction. This approach demonstrates the unprecedented concurrent realization of the unique reactivity among the methyl, methylene, and carbonyl groups in 3-methyl-5-pyrazolones for the construction of 2,3-dihydrooxepine rings. Moreover, a broad substrate scope displays a graceful diversity-oriented synthetic approach.