We survey the recent discovery of the oscillation of macroscopic variables in closed Hamiltonian dynamical systems with mean field coupling, in particular, the mean field XY model. This behavior is observed in a transient state during the relaxation to equilibrium, while the transient time is divergent with the system size. Periodic or quasiperiodic collective motion is sustained, even though each element shows chaotic motion. This collective motion appears through Hopf bifurcation, which is a typical route in low-dimensional dissipative dynamical systems. The collective oscillation is analyzed as a self-consistent solution between excitation of the swing-type mean-field motion and dynamics of each element driven by it. The universality of the phenomenon is also discussed.