The crystal structures of inorganic compounds comprising cationic complexes containing oxygen atoms coordinated tetrahedrally to metal atoms, or oxocentred groups [OM₄], are considered. The linking of the [OM₄] tetrahedra in the structures has been analysed and cationic complexes of different structures have been identified. The rules governing the linking of the [OM₄] tetrahedra have been formulated and the cationic complexes have been subjected to a detailed systematic treatment on their basis. Data on the statistics of the bond lengths and bond angles in the [OM₄] tetrahedra are presented.