Preheat oven to 350 degrees.
Rinse the chicken and pat dry with paper towels. Place the chicken, breast side down, onto a plastic cutting board. Tuck the wings behind the chicken for a neat presentation and to prevent the tips from getting overly browned. (Can also be done after removing the backbone.)
Starting at the neck, find the backbone. Cut along the backbone with good, sharp kitchen shears until you get to the tail.
Repeat on the other side of the backbone until you've removed it. (One spot near the thigh will require a little extra effort, but it can still be cut with kitchen shears.) Discard or freeze for stock.
Whisk together the lemon juice, olive oil, garlic, salt, and pepper in a small bowl.
Pour over the chicken, ensuring the chicken is evenly covered with oregano.
Place the lemon slices in the bottom of a 13 x 9-inch baking dish. Place the chicken--breast side up--over the lemon slices.
Ensure the wings are still tucked under the back and arrange the legs to come together at the chicken's claw (or foot). Tie with butcher's twine to keep the legs together while roasting. (If your baking/roasting pan is large enough, you can leave the legs spread out.)
Dot the top of the chicken with butter.
Roast for 1 to 1 1/2 hours or until an instant-read thermometer registers 165 degrees when inserted into the thickest part of the thigh. Baste with the lemony juices once or twice while roasting.
Let rest for 5 to 10 minutes then carve and serve.