Cytochrome p450 monooxygenases (CYPs) are a vast family of enzymes that play key roles in the biosynthesis of specialized metabolites. They are crucial catalysts involved in numerous biosynthetic reactions such as hydroxylations and oxidations. Monoterpenoid oxindole alkaloids (MOAs) are a subclass of monoterpenoid indole alkaloids (MIAs) and have known medicinal benefits. However, due to their low natural abundance and structural complexity, obtaining them in large quantities for study has been difficult. In this study, I purified and analyzed seven MOAs from Hamelia patens. I also identified and characterized a CYP responsible for catalyzing the oxidation of 3R heteroyohimbine-type MIAs to their MOA derivatives in Cephalanthus occidentalis. This discovery enables the de novo biosynthesis of major MOAs in C. occidentalis. The elucidation of this CYP and MOAs provides valuable insights into the enzymatic biosynthesis of MOAs and can enhance the production of these MOAs for future studies.