Crystal structures of inorganic compounds containing cationic complexes with oxygen atoms inside a tetrahedron of metal atoms, or oxygen-centered groups [OM₄], are reviewed. Types of a linkage of [OM₄] tetrahedra into the structures were analysed, and a number of the differently built cationic complexes were identified. Rules for a linkage of [OM₄] tetrahedra were formulated, and a detailed systematics of the cationic complexes in inorganic crystals was derived on their ground. A formation and a linkage of oxocentered tetrahedra in most of the crystal structures were considered for a first time. Statistics of bond distances and bond angles in [OM₄] tetrahedra was discussed.