I've done a few batches of really nice OMS now, based on a clone of Samuel Smiths Celebrated Oatmeal Stout. Basic %s are:
1051 OG
1014 FG
37 SRM
78% Maris Otter (or another English pale ale malt)
6% Crystal 120
6% Chocolate Malt
6% flaked oats, toasted in the oven on baking parchment at 165C until turning golden
4% Roasted Barley
60min boil, with 30 IBUs worth of East Kent Goldings at 60 mins
The best one I did was feremented with WLP004 Irish Ale at 20C . Superb mouthfeel; chocolatey, coffee, toffee, all that good stuff. I've converted several non-stout drinkers with this bad boy!
Oh, I did a 15 minute beta-glucan rest with the oats too - helps keep them from turning into porridge. Steep them at 45C with 30% of their weight of base malt in a saucepan or the like, then add to the mash.