Fusion cross sections with high precision have been measured for the O-16+ Ge-76 and O-18+ Ge-74 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 O-18 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.