In a large shallow bowl, mix together the olive oil, balsamic vinegar, garlic, oregano, 2 tablespoons chopped basil, salt and pepper. Add the chicken and allow to marinate for half an hour. In a large skillet over medium heat, pan-grill chicken for 5-6 minutes each side, or until completely cooked through in the center, and slightly charred on the outer edges. While chicken is cooking, grill the tomato slices for 1-2 minutes each side. Spread each chicken breast with a spoonful of fresh pesto, then top with 1 slice of mozzarella and 1 grilled tomato slice, allowing the heat to melt the cheese. (If needed, place the oven-safe skillet in the oven and heat at 350 degrees for 2-3 minutes to melt the cheese.) To serve, top with the shredded basil and balsamic reduction.