A process for preparing Tyrosol and/or Hydroxytyrosol from oil mill wastewaters. includes: a) Microfiltration (MF), Ultrafiltration (UF), Nanofiltration (NF) and Reverse Osmosis (RO) of the OMW; b) Separation of Tyrosol, Hydroxytyrosol and other phenolic compounds from the concentrated RO; c) Oxidation of the obtained Tyrosol to Hydroxytyrosol in the presence of methyl rhenium trioxide and of hydrogen peroxide in a protic solvent.