Slow Cooker Shredded Chicken

1 tablespoon extra-virgin olive oil
1 pound boneless, skinless chicken thighs, cut into 1 and 1/2-inch pieces
1 pound boneless, skinless chicken breasts, cut into 1 and 1/2-inch pieces
1 medium yellow onion, diced small
3 garlic cloves, roughly chopped
1 medium red bell pepper, seeded and diced small
1 can (14.5 ounces) crushed tomatoes
1/4 cup hot-pepper sauce, such as Frank’s
3 tablespoons Worcestershire sauce
2 tablespoons yellow mustard
1 tablespoon unsulfured molasses
8 hamburger buns