Octahedron rhymes with:
tetrahedron, polyhedron, tetrahedron, polyhedron, tetrahedron, audran, cauldron, chadron, children, condron, excedrin, gendron, grandchildren, hendron, hydron, kindran, madron, padron, philodendron, polyhedron, ravindran, rhododendron, schoolchildren, squadron, stepchildren, tetrahedron, vorndran, waldren, waldron
Octahedron sounds like:
osterman, ostermann, ostrem, ostrom, ostrum