In a resealable gallon-sized bag, mix together olive oil, ranch dressing, worcestershire sauce, rosemary, salt, lemon juice, vinegar, and pepper. Add in chicken breasts, close zipper, and place bag in the fridge to marinate for 2-8 hours (I recommend letting it marinate for as long as possible).
Remove chicken from fridge and discard marinade. Preheat grill over medium high heat and cook chicken breasts for a couple minutes on each side, until completely cooked through (internal temperature reaching 165 degrees F).
Notes
You can also use dried rosemary in place of the fresh. Just add in 1/2 - 1 teaspoon dried, but fresh tastes best!