  Examination of high-resolution, experimental fusion excitation functions for
$^{16,17,18}$O + $^{12}$C reveals a remarkable irregular behavior that is
rooted in the structure of both the colliding nuclei and the quasi-molecular
composite system. The impact of the $\ell$-dependent fusion barriers is
assessed using a time-dependent Hartree-Fock model. Barrier penetrabilities,
taken directly from a density-constrained calculation, provide a significantly
improved description of the experimental data as compared to the standard
Hill-Wheeler approach. The remaining deviations between the parameter-free
theoretical mean-field predictions and experimental fusion cross sections are
exposed and discussed.
