Preheat the oven to 350 degrees F (175 degrees C). Place a roasting rack on a rimmed baking sheet.

Place turkey, breast-side down, on a cutting board. Using a pair of sharp heavy-duty kitchen shears, cut along one side of backbone.

Repeat on the other side of backbone. Remove backbone and reserve for making turkey stock for gravy.

Press firmly down onto both sides of turkey breast to flatten.

Put turkey, breast-side up, on the roasting rack. Tuck wing tips behind breast. Pat skin dry; rub oil over entire turkey. Season with sage, thyme, rosemary, salt, and pepper.

Roast in the preheated oven for 1 hour 30 minutes, rotating the baking sheet every 30 minutes.

Increase the oven temperature to 400 degrees F (200 degrees C); continue roasting until skin is crisp and golden, about 15 minutes more. An instant-read thermometer inserted into the thickest part of the thigh should read 165 degrees F (74 degrees C).

Remove turkey from the oven, cover loosely with a double sheet of aluminum foil; let rest, 10 to 15 minutes before slicing.