Double octahedron
A double-sided octahedron contains six octahedra of unit sides and holes. Eight regular tetrahedra of side one fit in there.
This shows that the volume of the large octahedron, which is equal to eight small octahedra, is equivalent to that of six octahedra and eight tetrahedra. So four tetrahedra are worth one octahedron.
Find these volume equalities analytically (good luck!).
A YouTube playlist implementing these decompositions with origami.