The effect of buoyancy ratio on the flow structure is investigated numerically for a binary mixture gas in a rectangular enclosure subject to opposing horizontal thermal and compositional buoyancies. The following conditions were considered : Ra-T = 10(5), Pr = 1, Le = 2 and N = 0.0-2.0 for A = 2. The numerical solution predicts that oscillatory double-diffusive convection with the secondary cell flow structure occurs for a certain range of buoyancy ratio. The key mechanism for oscillatory flow is that the unstably stratified region of species shifts from the central part of the enclosure to the upper and lower parts, and vice versa in a time-periodic sense, due to the interaction of heat and mass transfer with different diffusivities near the vertical walls. Bifurcation structures of the oscillatory flow in the present system are discussed.