Heat olive oil in heavy large pot over medium heat. Add onion, carrot, celery, garlic, and oregano. Cook, stirring frequently, until vegetables soften, about 10 minutes. Add salt, chicken broth, tomatoes with their juice, and chipotles. Raise heat to medium-high and bring mixture to a boil. Reduce heat and simmer, uncovered, about 30 minutes.

Puree soup in a food processor (in batches), or using an immersion blender until smooth. Return soup to pot, stir in corn and evaporated milk, and raise heat again to medium-high. Cook until soup is hot and close to boiling, about 5 minutes. Stir in lime juice. Serve soup topped with chopped cilantro and avocado.