Fusion cross section data for the O-16,18,(20) + C-12 systems at energies near and below the Coulomb barrier are studied within the framework of the optical model. To examine these reactions, the microscopic nucleon-nucleon double folding potentials for real and imaginary parts are found. To make a comprehensive analysis of the effect of neutron-rich systems, we used three different reactions from stable to unstable systems. The microscopic nucleon-nucleon double folding potentials show very good agreement for O-16(,)18(,20) + C-12 systems with a very weak imaginary potential. Also, we have obtained better agreement by using fully microscopic nucleon-nucleon double folding potentials with the O-20 + C-12 system's experimental data than in previous works, but especially at low energies we still have some problems reproducing the O-20 + C-12 system's fusion cross section data.