Chocolate stout.

It usually is just the chocolate malts, dont go too big on the dark chocolate malts, how much to use is hard to say, it can depend on the rest of the recipe, you might use 100g-500g it's going to take some experimenting to find out what you like. There is light chocolate malt which might help too, I dont have much experience with those.
Some will also add chocolate essence, cacao beans, cocoa, real chocolate.
Chocolate oatmeal stout really isn't a standard recipe, BJCP so its really up to you what you want to do and which direction you want to go.
I'd be adding a generous amount of oats anywhere from 500g to 1.5kg, you might want lactose as well, or maltodextrin for some added body.

you might start with something like
share.brewfather.app/O1zrb1t4MUTmpi

Cheers!

Just remember not all chocolate malts are equal, some are twice as strong as others, so you have to keep an eye on the colour numbers EBC

It will be an issue if you a substitute and don't take note of the EBC, and its a 2x as strong dark chocolate malt and your beer ends up tasting like burnt grain or something. All I'm saying is all chocolate and roasted malts are not equal even if they have the same name. Its not just about colour (EBC) but its a great indication of the malt characteristics when you're substituting. Cheers!