Catalyzes the oxidation of the methyl group to a carboxyl group at the C-28 position of beta-amyrin to form oleanolate. beta-amyrin + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = 4 H(+) + 4 H2O + oleanolate + 3 oxidized [NADPH--hemoprotein reductase] Belongs to the cytochrome P450 family.