Heat a large skillet over medium. When the skillet is warm, add the oil and onion. Sauté for 4 minutes or until tender, stirring occasionally. Add the garlic and cook for 30 seconds more. Stir in the chipotles, oregano, and cumin, and cook for 1 minute. Add in the tomatoes. If you're doing fresh tomatoes, let them cook down a bit, then add the stock and salt. If you're using canned tomatoes, add the stock, and salt at the same time. Bring to a simmer, and cook for 7 minutes.