Oatmeal is generally water soluble, but even then, it doesn't completely dissolve. In any case, I don't think you can fix it as a butter. You might be able to use the butter in something else though...maybe in a scrub?
I've never posted before but I have experience with making butters using colloidal oatmeal and have never encountered a grainy texture. I almost feel that it is most likely your shea butter causing the graininess and would suggest melting your butter down in a double boiler to 175-180 and holding it there for at least 15 minutes, stirring occasionally to ensure all the fine grains are melted and then cool fast by placing in the fridge or freezer. I have found this the best method to prevent the graininess that can sometimes happen with shea butter.