Cut the chicken breast halves into 2 equal pieces. Place the pieces of chicken between two sheets of waxed paper and flatten them with a rolling pin or meat mallet. Combine olive oil, garlic, oregano, lemon zest, thyme, salt and pepper in a large zip-lock plastic bag and mix well. Add all pieces of chicken and massage marinade into meat until evenly coated. Seal the bag and marinate in the fridge for at least 1 hour, turning occasionally.