First, measure out all ingredients and have them ready.
If you don't wish to marinate your tomatoes, skip steps 2 & 3. Combine olive oil, vinegar, garlic, salt. and oregano in a medium bowl to create the marinade. Whisk to combine.
Add sliced tomato to the marinade, cover, and refrigerate for at least 2 hours and up to overnight. Make sure to take the tomatoes out of the fridge at least 30 minutes before making the sandwich so the oil can reincorporate with the rest of the marinade.
Place bacon in a medium frying pan over medium heat. Cook until the bacon is crisp or to your liking - about 2 minutes per side. Place the cooked bacon on paper towels to soak up additional grease. Do not drain the bacon grease from the pan.
If your chicken is thicker than approximately 1 inch, slice the chicken on the bias to create 1 inch medallions. Season both sides with salt and pepper to taste.
Increase the heat on the frying pan to medium-high. If there is not adequate bacon grease to cook the chicken in, add a small amount of neutral oil, such as grapeseed or avocado oil.
Once hot, add the chicken and cook for 3-4 minutes per side, until the chicken reaches an internal temperature of 165 degrees Fahrenheit. Let the chicken rest on a cutting board while you assemble the sandwich.
Toast the bread either in a toaster or in the pan used to cook the chicken and bacon.
Assemble the sandwich. Spread mayonnaise on one side each of the toasted bread. Then add the chicken, bacon, lettuce and tomato. Drizzle some of the marinade on the interior of the bread to amplify the flavor.