Purpose: The effect of solid-state fermentation (SSF), employing different microbial strains (single or co-cultured), to the chemical composition of olive leaves (OL) and the possible perspectives of the derived material for high added-value applications was explored. Methods: Emphasis was given on bioactives (oleuropein, OLE, hydroxytyrosol, HT, elenolic acid (EA) related compounds, maslinic (MA) and oleanolic (OA) acids). In parallel, the levels of other chemical components with nutritional/antinutritional interest for feed application and certain minerals were also measured. Results: A gradual decrease in OLE and an EA derivative till their complete loss was found. HT progressively increased and then consumed reaching low levels. MA and OA were unaffected. A. niger resulted in the highest formation of HT (1 mg/g dw), and the lowest loss of OL antioxidant potential (13.8% at 72 h). Varying levels of protein production were observed potentially improving their nutritional value. Conclusion: The study demonstrated that fermented OL significantly altered phenolic compounds, particularly OLE and HT, and maintained triterpenic acids such as MA and OA. Despite reductions in certain phenolics, fermented OL showed improved nutritional profiles, particularly in protein content and antioxidant potential, suggesting their potential for added-value applications in various industrial sectors, including animal feed. To our knowledge this is the first time that the co-cultures selected in the present study were employed for OL SSF and that under all conditions examined the triterpentic acids MA and OA were the dominant bioactives despite some improvements in HT formation Graphical Abstract: (Figure presented.)