Slow Cooker Chili



1 tablespoon olive oil
2 pounds lean ground beef
1 large onion, chopped
4 garlic cloves, minced
2 (28 ounce) cans crushed tomatoes
½ cup water
2 teaspoons dried oregano
1½ tablespoons chili powder
1½ tablespoons ground cumin
2 teaspoons cocoa powder
2 teaspoons chipotle chili powder (you can substitute chilies in adobo sauce as well. Add to taste)
salt and pepper, to taste
1 (15 ounce) can dark red kidney beans, drained and rinsed
1 (15 ounce) can black beans, drained and rinsed

For serving:
shredded cheddar cheese
sour cream
green onions
tortilla chips