The subbarrier fusion process is studied for systems involving oxygen isotopes. A one-channel incoming wave boundary condition (IWBC) calculation gives an excellent fit to fusion cross section data for 16O+16O. An IWBC coupled channels calculation for 17O+16O that includes inelastic excitations as well as one-neutron transfer with formfactors calculated in a consistent single-particle framework reproduces the subbarrier enhancement down to four fifths the barrier height, but not below that. The calculation does not invoke the adiabatic approximation, which would create non-unitarity in the coupled channels equations. The measured subbarrier fusion cross section for 18O+16O is well reproduced by an IWBC coupled channels calculation with two-neutron transfer, but the calculation disagrees with the above-barrier data.
