Catalyzes the C-28 oxidation of beta-amyrin to form erythrodiol (PubMed:25578277). Catalyzes the C-28 oxidation of erythrodiol to form oleanolic aldehyde (PubMed:25578277). Catalyzes the C-28 oxidation of oleanolic aldehyde to form oleanolate (PubMed:25578277). beta-amyrin + O2 + reduced [NADPH--hemoprotein reductase] = erythrodiol + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] erythrodiol + O2 + reduced [NADPH--hemoprotein reductase] = H(+) + 2 H2O + oleanolic aldehyde + oxidized [NADPH--hemoprotein reductase] O2 + oleanolic aldehyde + reduced [NADPH--hemoprotein reductase] = 2 H(+) + H2O + oleanolate + oxidized [NADPH--hemoprotein reductase] Belongs to the cytochrome P450 family.