Will this make natural yogurt curdle?

Yogurt is already curdled, that's what it is, milk protein that has been curdled by bacteria that turned the milk sugar to an acid. You cannot use yogurt instead of water or milk to cook oatmeal because oats need to absorb a lot of the liquid to cook properly, and yogurt is already thickened and will cause the mixture to be a thick lump of partially cooked oats. Oats contain a lot of starch, and that starch needs time and heat to absorb the liquid to become soft (gelatinize). You can add yogurt to the oatmeal after it has cooked, however, since the oats will have already become soft and smooth. If the yogurt contains an active culture (described on the label as such), then it's healthier to not heat it up because that will kill the good bacteria (lactobacillus acidophilus) that helps your stomach and intestines, This bacteria is called a probiotic, because it's good for you.