Fusion cross sections with high precision have been measured for the 16 O+76 Ge and 18 O+74 Ge systems at energies near and below the Coulomb barrier. Coupled-channels calculations including inelastic excitations well reproduce the experimental fusion excitation functions and the corresponding barrier distributions. There is no evidence that the positive Q -value two-neutron transfer plays a role in the fusion process. Systematic investigations of 18 O induced fusions, whose data are available in the literature, have been performed to understand the role of two-neutron transfers with positive Q -values in the fusion process, but results are inconsistent and no conclusion can be made. The effect of two-neutron transfer with positive Q -value on sub-barrier fusion is still an open question, which becomes a challenge and requires further efforts both experimentally and theoretically.