A new distinct cultivar of Osteospermum plant named `Sunny Plum Serena`, characterized by its unique red-purple color combination of the upper and underside of the ray florets: (when opening) upperside RHS 72B, with stripes of RHS 155A, white, and underside RHS 72B; (fully opened) upperside and underside RHS 72C. Fading to RHS 199C, gray-brown; upright, compact and globular plant habit; dense and bushy growth habit, mainly due to upright stems; moderately vigorous growth habit, but less need for chemical growth retardation; and high number of inflorescences per plant.