The problem of zero-dispersion nonlinear resonance - a phenomenon that can occur in a periodically-driven nonlinear oscillator whose eigenfrequency as a function of energy possesses an extremum - has been formulated in general for both the dissipative and nondissipative situations. A complete bifurcation analysis and classification of period-l orbits is presented. The significance of bifurcations for the onset of chaos in the system, and for fluctuations in the presence of external noise, is discussed.