I am planning an oatmeal stout and read that the mash pH is really important for stouts in general. Which pH should I go for in my oatmeal stout and how can I get to it?
You want to make sure that your MASH pH stays in the proper range of 5.2-5.5. THis is important for proper enzymatic activity in the mash. Dark roasted malts and drop the pH as they tend to be more acidic than lighter malts. That drop of pH can effect the enzymes responsible for starch to sugar conversion.
pH is not something that is a concern for flavor of an oatmeal stout specifically.
If you suspect you are going to have pH issues (due to some weird brewing water chemistry) then you need to adjust your mash process. The easiest thing to do would be to mash the grist minus the dark malts. Then during the sparge mix the crushed malts into the surface gently to get them wet. Then sparging over the top of the grain bed will pull the color and flavors through the mash. At that point the pH is less of a concern because the conversion is done.
Mash pH should be 5.2-5.5 measured at room temperature. Room temp is the standard reference for pH. You can adjust pH with mineral additions. Dark grains have a tendency to drive down the pH and the darker the grain the more that will happen. Chalk (CaCO3) is often used to raise pH for dark beers. The problem with that is that it's only partially soluble so you may not be able to add enough to get the pH where you need it without adversely affecting the flavor. A better choice is pickling lime (Calcium hydroxide-Ca(OH)2). It has much better solubility and is much more effective at raising pH.