The nonrelativistic expansion of the mass operator of the Leutwyler-Stern covariant oscillator equations supplemented by the short distance gluon exchange contribution has been performed. The resulting operator of the Fermi-Breit type reveals a rich spin—dependent structure which might be suitable for the description of quarkonium. Its energy spectrum of order (v/c)2 is compared to the fully relativistic spectrum of the original mass operator. The differences for higher lying states of charmonium (e.g. ψ′) are as large as 100 MeV, demonstrating the importance of higher-order relativistic effects. The non-relativistic bound state equation with long-range confining and spin-dependent forces and short-distance Coulombic terms is proposed and solved numerically.