Optical density is the degree to which a refractive medium delays transmitted rays of light. But Refractive medium itself contains the Optical Density.
Actually Optical density is the density through which an object can be viewed. Actually it is the degree of the density to which a refractive medium( mirror or some transparent objects) will slow down the speed of the light rays which are transmitted. And the Optical Density lies inside the Refractive Medium itself