In a large skillet over medium-high heat, add the oil. Saute the onion for 3 to 5 minutes or until softened. Add the garlic and oregano and saute for 1 minute. Add the tomatoes and their juice, chipotle, and broth and bring to a boil. Reduce the heat and simmer for 20 minutes or until thickened, stirring occasionally. Season with salt and pepper and additional chipotle if desired. Transfer to a blender or food processor and let cool slightly before pureeing until smooth.
Meanwhile, sprinkle the chicken with salt and pepper. In the same skillet, over medium-high heat, c ook the chicken for 3 to 4 minutes per side. It will not be cooked through.
Transfer the chicken to a baking pan and top with pureed tomato sauce. Bake for 45 minutes or until the chicken is tender and the sauce has thickened. Remove the chicken to a cutting board, reserving the sauce. When cool enough to handle, use your finger or two forks to shred into small pieces. Return the chicken to the sauce and stir to combine. Serve with tortillas or rice.