Plant extracts with stabilizing and emulsifying qualities are commonly used in nanotechnology for the eco- friendly production of metallic nanoparticles. The study focused on synthesizing CoO and ZnO nanoparticles by the use of rosemary leaf extract. Quantum chemical calculations were performed using the DFT method to determine the active sites of the main chemical compounds in rosemary extract that catalyze the bi-reduction of Co+2 and Zn+2, resulting in the eco-friendly production of CoO and ZnO nanoparticles. Rosemary leaf extracts include a significant quantity of hydroxyl groups (–OH), oxygen atoms as heteroatoms, and many aromatic rings. The enhanced amount o π-electrons, resulting from aromatic rings and electron-releasing groups connected to these rings, acilitates the simultaneous reduction of Co and Zn metals. This happens because the π-electrons are more likely to interact with the vacant d-orbitals of Co and Zn. Antibacterial activity tests show that CoO and ZnO nanoparticles manufactured using green methods are effective against both gram-negative and grampositive bacteria, as well as the fungal isolate C. albicans with superior activity of CoO nanoparticles. The MIC assay results show that the synthesized nanoparticles are more effective against gram-negative E. coli bacteria than gram-positive S. aureus bacteria and C. albicans. CoO nanoparticles showed about 1.5x - 3x fold more antibacterial activity compared to ZnO nanoparticles. The results emphasize the possibility of using green synthesis o nanoparticles as an environmentally benign and cost-efficient technique for mass manufacturing, showing potential applications in the fields of medicine and industry.