Summary: We consider a large number \(N\) of quantum particles coupled via a mean field interaction to another quantum system (reservoir). Our main result is an expansion for the averages of observables, both of the particles and of the reservoir, in inverse powers of \(\sqrt{N}\). The analysis is based directly on the Dyson series expansion of the propagator. We analyse the dynamics, in the limit \(N\rightarrow\infty\), of observables of a fixed number \(n\) of particles, of extensive particle observables and their fluctuations, as well as of reservoir observables. We illustrate our results on the infinite mode Dicke model and on various energy-conserving models.