A new distinct cultivar of Osteospermum plant named 'Sunny Stephanie', characterized by purple ray floret color from RHS N78A with stripes of RHS N77B purple to RHS 59C, red-purple, with stripes of RHS N57B red-purple; dense and bushy plant form, mainly due to more upright stems; moderately vigorous growth habit, less need for chemical growth retardation; and high number of inflorescences per plant.