Heat oil in a skillet over medium-high heat. Stir flour and chili powder into hot oil, reduce heat to medium, and cook until the flour is lightly brown in color, 3 to 5 minutes.
Gradually stream water and tomato sauce into the flour while stirring until the flour is completely incorporated into a smooth mixture. Stir oregano, cumin, garlic powder, and onion salt into the liquid. Continue cooking until thickened slightly, about 10 minutes; season with salt.