Catalyzes the carboxylation of beta-amyrin at the C-28 position to form oleanolic acid. May be involved in saponin biosynthesis in fruit skin. beta-amyrin + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = 4 H(+) + 4 H2O + oleanolate + 3 oxidized [NADPH--hemoprotein reductase] Expressed in leaves, stems and fruit skin. Belongs to the cytochrome P450 family.