We report a facile and generic method for the synthesis of hollow mesoporous silica nanoreactors (HMSNs) with small‐sized metal oxide nanoparticles (NPs) inside their cavities. They were made by deposition of silica onto metal‐containing charge‐driven polymer micelles and subsequent calcination. The micelles consist of 1) negatively charged supramolecular polyelectrolyte chains of bis‐ligand‐bound metal ions, and 2) water‐soluble, neutral/positive diblock copolymers. Owing to the facile coordination between transition‐metal ion and the employed bidentate ligand, a series of HMSNs with <2 nm MxOy NPs inside cavities (M=Mn, Co, Ni, Cu, or Zn) were obtained by simply varying the metal ions inside the micelles. The developed method circumvents the pre‐ and post‐synthesis of metal oxide NPs; after calcination, hollow mesoporous nanostructures containing small‐sized metal oxide NPs inside their cavities are directly obtained. The CoxOy‐functionalized HMSNs catalyze the degradation of various dyes with H2O2.