A simple, one-step synthetic route to prepare ordered mesoporous silica monoliths with controllable quantities of metal oxide nanocrystals in their channels is presented. The method is based on the assisted assembly effect for mesostructure-directing of the metal complexes formed by the interaction of metal ions with the –O– groups of copolymers. Highly ordered hexagonal silica monoliths, loaded with various metal oxide nanocrystals, including those of Cr2O3, MnO, Fe2O3, Co3O4, NiO, CuO, ZnO, CdO, SnO2, and In2O3, can be obtained by this one-step pathway. In the NiO/SiO2 nanocomposite, nickel oxide nanorods with face-centered cubic lattices are formed at low doping ratios, and they can be transformed into nanowires by increasing the quantities of the precursors. In the Fe2O3/SiO2 nanocomposites, a one-dimensional assembly of iron oxide nanoparticles is observed. In the In2O3/SiO2 nanocomposites, single crystal nanowires with high aspect ratios are obtained. For the other metal oxide nanocomposites, including Cr2O3, MnO, Co3O4, CuO, ZnO, CdO, and SnO, only crystalline nanorods are obtained. N2 sorption results of the metal oxide/SiO2 mesostructured nanocomposites reveal that nanocrystals inside the pores do not severely decrease the pore volume or the Brunauer–Emmett–Teller (BET) surface area of the mesoporous silica host. The bandgaps of SnO2 and In2O3 nanocrystals, calculated from UV-vis spectra, are much larger than the corresponding bulk materials, implying the quantum confinement effect in the small particles. Co3O4/SiO2 mesostructured nanocomposites catalyze the complete combustion of CH4. These studies provide a new and simple method for templating synthesis of metal oxide nanostructures.