Place chicken in a gallon-size bag or large bowl. In a separate bowl, whisk olive oil, garlic, lemon juice, orange juice, mustard, oregano, salt, and pepper; drizzle all but 2 tablespoons of the marinade (reserve for potatoes) over chicken. Marinate chicken for at least 20 minutes to 1 hour. Drain the chicken from the marinade, discarding the marinade.