In large resealable plastic bag, combine salt, pepper, oregano, cumin, garlic, orange juice, lime juice and onion slices. Seal bag and shake to mix well. Add chicken, seal, shake well and refrigerate at least 2 hours and up to 24 hours.
Remove chicken from marinade and pat dry. In a large skillet over medium-high heat, warm olive oil. Add chicken and brown on all sides, turning frequently, about 10 minutes total. Reduce heat to medium-low, cover skillet and let chicken cook 10 minutes more. Remove chicken and set aside on a plate.
Pour marinade with onions into same skillet; increase heat to medium. When sauce simmers, return chicken to pan, along with any juices that have collected on the plate. Cover skillet, reduce heat to very low and simmer for 20 minutes.