Preheat oven to 400º and grease a large baking sheet.
Arrange tenderloin on the pan with orange slices under the edges and season generously with salt and pepper.
In a medium saucepan, bring orange juice, brown sugar, garlic, and Italian seasoning to a boil, then reduce heat and simmer until reduced by half, 5 to 7 minutes.
Pour mixture over pork.
Bake for 25 minutes, then switch to broil and cook 5 minutes more.
Slice and serve immediately.
