My oatmeal stout recipe has a distinct "coffee" flavor to it, instead of the traditionally "roasty" stout. I don't use lactose (I dislike sweet things) but I was thinking we could compare our recipes since I use oatmeal as well.
I don't really like the strong roast of roasted barley, since I like the coffee flavored roastiness of Briess black malt instead. I think that is where I get the coffee flavor from, without even using real coffee. I don't even use roasted barley in it, but I think in your recipe 12 ounces might be a bit much if you want a real coffee note.
Here's mine (from this thread: http://www.homebrewtalk.com/f68/yoopers-oatmeal-stout-210376/):
7 lbs Pale Malt, Maris Otter (3.0 SRM) Grain 63.64 %
1 lbs Oats, Flaked (1.0 SRM) Grain 9.09 %
12.0 oz Victory Malt (25.0 SRM) Grain 6.82 %
10.0 oz Chocolate malt (pale) (200.0 SRM) Grain 5.68 %
8.0 oz Barley, Flaked (1.7 SRM) Grain 4.55 %
8.0 oz Black Barley (Stout) (500.0 SRM) Grain 4.55 %
8.0 oz Caramel/Crystal Malt - 80L (80.0 SRM) Grain 4.55 %
2.0 oz Chocolate Malt (450.0 SRM) Grain 1.14 %
2.00 oz Williamette [4.80 %] (60 min) Hops 31.6 IBU
1 Pkgs British Ale II (Wyeast Labs #1335)
The victory malt gives some nice "toasty" notes, while the pale chocolate malt gives great flavor. The flaked barley is for head and head retention. The small amount of crystal malt 80L is for some residual sweetness. Adding lactose to a recipe like this would really be creamy and semi-sweet.