Anti-Coagulant for Oat Milk
Heating oatmilk basically works on the same principle as heating any other thing with starch like a roux or thickening soup/gravy with cornstarch. 

To get less thickening, add less starch i.e. less oats content per liter of water. To compensate for lack of flavor blend it with any other type of plant based milk, like soymilk (protein-based) or cashew milk(protein+fat based), or anything which is not primarily starch based. It will add nutritional value, compensate for watered down flavors (due to less oats and more water) and will not thicken upon heating.

The only other way is to use chemical additives.