If you are just looking to increase the ABV keeping the basic recipe the same, then you can click on the OG slider below the ingredient window on the design tab and plug in the desired OG. The program will scale all the ingredients to meet this increased gravity specification. The intent when using this feature is to keep the basic recipe the same in terms of balance of ingredients while just proportioning them to the new target.
If you want to add some other ingredient, then it is best to do that up front. For adding oats, oat malt (in which case you will be doing a partial mash) or golden naked oats (caramelized oat malt) will work. Flaked oats will supply starches, but you will need to mash these to convert them to sugars.
Another way to increase the body is to use some Carapils or Carafoam with your steeping grains or find a yeast with a lower attenuation to leave more unconsumed sugars behind.
Of course, just increasing the initial amount of fermentables in the recipe will bring up the final gravity as well, so you might just want to make that change first, brew and then make further adjustments on the higher gravity version. I find it easier to make a single adjustment that I can follow so that when comparing how the beer changed from the first brew, I know what adjustment caused that difference and can apply that understanding to the next recipe I adjust.
