HPLC analysis shows that stored OMW is greatly enriched in phenolic compounds, mainly hydroxytyrosol (HT) (2.54 g L⁻¹) and tyrosol (0.775 g L⁻¹). But, after 4 years these compounds decrease in concentration. Potentiodynamic polarization measurements show that OMW acts as a mixed-type corrosion inhibitor for copper in 3 wt% NaCl solution. The exceptional effect of this inhibitor is above E_corr. The full passivation range greatly increases upon the addition of inhibitor and increases further with increasing OMW concentration. For sufficiently-high OMW concentrations, a notable absence of trans-passivation phenomenon until 2500 mV is detected and no pits are observed on the samples’ surface.