We consider an analog of particle production in a quartic O(N ) quantum oscillator with time-dependent frequency, which is a toy model of particle production in de Sitter space and dynamical Casimir effect. We calculate exact quantum averages, Keldysh propagator, and particle number using two different methods. First, we employ a kind of rotating wave approximation to estimate these quantities for small deviations from stationarity. Second, we extend these results to arbitrary strong deviations using Schwinger-Keldysh diagrammatic technique. We show that in strongly nonstationary situations, including the case of resonant oscillations, loop corrections to the tree-level expressions result in an additional degree of freedom, N → N + 3 2 , which modify the average number and energy of created particles.