Arnica oil can be used as a massage oil to stimulate the body and soothe muscle pain. Athletes often use arnica oil to sooth over tired and strained muscles and joints. Arnica oil is a well known herbal remedy for bruises, sprains and muscles strains. Apply a small amount of the oil and massage into the affected area to reduce bruising after an injury, or to treat muscle soreness after injury or exercise. As with any new herbal remedy, be sure to spot check on a small patch of skin to check for allergies.