The evolution of open systems at late times differs fundamentally from that of closed systems. While the latter thermalize to a temperature determined by the energy density of the initial state in the former, a competition between this mechanism and equilibration with the environment emerges. In this talk, I will derive the dynamic theory in the symmetry-broken phase of an O(N)-symmetric field theory based on an expansion of the two-particle irreducible (2PI) effective action to next-to-leading order in 1/N and highlight the analogies and differences to the corresponding theory for closed systems. This approach culminates in the open-system Boltzmann equation valid in the absence of quasi-particles. Specifically for the O(N) model interactions are screened efficiently at small momenta and, therefore, the late-time evolution is effectively collisionless.