In this paper, we investigate a two-dimensional isotropic harmonic oscillator on a time-dependent spherical background. The effect of the background can be represented as a minimally coupled field to the oscillator's Hamiltonian. For a fluctuating background, transition probabilities per unit time are obtained. Transitions are possible if the energy eigenvalues of the oscillator Ei and frequencies of the fluctuating background ωn satisfy the following two simple relations: Ej ≃ Ei − ℏωn (stimulated emission) and Ej ≃ Ei + ℏωn (absorption). This indicates that a background fluctuating at a frequency of ωn interacts with the oscillator as a quantum field of the same frequency. We believe this result is also applicable for an arbitrary quantum system defined on a fluctuating maximally symmetric background.