Alkaloid extracts of leaves (OOL) and bark (OOB) of Ochrosia oppositifolia, as well as isoreserpiline (ISR), the major alkaloid isolated from OOL and OOB, were investigated as potential corrosion inhibitors for mild steel (MS) in 1 M HCl medium. The inhibition properties of these phytoconstituents were studied using electrochemical techniques (potentiodynamic polarization measurements and electrochemical impedance spectroscopy, EIS) and scanning electron microscopy (SEM). The results indicated that these green inhibitors effectively reduced the corrosion rate. Polarization studies showed that these inhibitors decreased the corrosion current densities by a mixed-mode mechanism. The EIS data were analyzed by an equivalent circuit model for the electrode/electrolyte interface. SEM observations confirmed the existence of an adsorbed protective film of green inhibitors, and the adsorption was found to follow the Langmuir adsorption isotherm. FTIR and molecular modeling studies were also employed and revealed that the presence of ISR could be responsible for the corrosion inhibition potential of OOL and OOB.