Oatmeal really needs to be mashed, without the ezymes from basemalt it will just add a bunch of starches to your beer.
Here is an all-grain recipe that made a great beer. Just swap out any pale malt you can't fit for light DME.
6 lbs Golden Promise (any pale malt would work)
.75 lbs Oatmeal (Quaker Quick)
.5 lbs CaraMunich III (any medium-dark crystal would work)
6 oz of Carafa 1
4 oz Black Patent
6 oz of Pale Chocolate
Adjust your water so it has enough carbonate to buffer the dark malts.
Mash at 154
90 min boil
Enough hops (I used nugget) at 60 minutes to get to 24 IBUs.
Ferment around 65 with American Ale yeast (001/1056)