Blue tetrahedra sit over openings between the octahedra and correspond to the purple tetrahedra in the next layer down. Two layers are shown and part of a third so that the layering sequence becomes apparent. All the cations occur at the centers of octahedra or tetrahedra. Although the oxygens are close-packed, olivine is orthorhombic because of the way the cations fill the voids between the oxygen sheets. There are actually two kinds of octahedra.
Every oxygen links to one silica tetrahedron and three ferromagnesian octahedra.
Sometimes it's easier to do this sort of accounting if we pair specific anions and cations together rather than distribute the charges among neighbors. Note that neither diagram shows a tetrahedron meeting three octahedra. We'd have to show additional layers or build a three-dimensional model. But we can reason out that it must be so from Pauling's Second Rule. The vacant points of tetrahedra in the polyhedral view would connect to the joins between three octahedra in the next layer.