Heat a large skillet over medium-high heat, then add the oil.
Once the oil is hot, add the chopped onion and sauté until it becomes tender, which should take about 5 to 7 minutes.
Add the garlic, chile powder, and oregano, and continue sautéing until the fragrance is released, about 1 to 2 minutes more. Add the beans and tomatoes along with a few generous pinches of salt.
Let the mixture simmer until the tomatoes break down, typically around 20 minutes.
Taste the chili and adjust the seasoning as needed by adding more salt, chile powder, and oregano according to your preference. Serve the chili with the pickled onions and any garnishes of your choice.