Preheat oven to 350°F.
Line sheet pan with parchment paper; place turkey breast on parchment paper. Roast until internal temperature reaches 165°F. Let stand for 20 minutes.
While turkey is standing, melt butter in sauté pan over medium heat; add celery and onion and cook for 2 minutes. Add orange juice and chestnut purée; stir until mixture is smooth. Add dark rum; cook for 5 to 8 minutes or until thickened. Remove from heat; fold in pumpernickel bread cubes, orange segments and chestnuts.
Slice roast turkey; place turkey slices on serving plate. Spoon stuffing mixture over turkey slices.
Garnish with oregano sprig, if desired. Serve immediately.