I don;t remember gearoil being forced up there, but if overfilled, overheated, and over-stirred, it might-could. Perhaps just lower the level and see if it persists? Can you see a crack in the housing when it's all cleaned up?
I see one O-ring in the parts list under the flange that screws into the gearbox itself. Yours appears to be leaking from higher up on the housing.