I am by no means an expert on Porter, but I will tell you that my dark beers got better once I understood that those very dark roasted grains (chocolate, roast barley, etc) can pull down your mash pH too low, and give off chalky, astringent, dry flavors.
To get around this, I suggest cold-steeping your dark grains separate from your mash (or steep, if you are doing extract). Soak the grains in room temp water for a few hours or up to day, and then strain and pour the liquid into the boil fairly near the end. You'll get the color of the grains, and some flavor, but not as much as if you mashed them.
Oats in a Porter sound like a good idea, since their creaminess might help cut any acidity, but I'm not positive. Other things I know would help would be (1) low color Crystal Malts (10-60) to add some creamy caramel flavors, or (2) Malto Dextrin, which would increase the body of the beer (so don't add too much, Porter should be drinkable). Both those would cut the acidity slightly.