It is known that fiber nonlinearities induce crosstalk in a wavelength division multiplexed (WDM) system, which limits the capacity of such systems especially at higher signal powers. Traditionally, the channel capacity of a single WDM user is analyzed under different assumptions for the transmitted signals of the other users. In this paper, we instead take a multi-user information theoretic view of an optical WDM channel impaired by cross-phase modulation and dispersion as an interference channel. We characterize, for the first time, an outer bound on the capacity region of simultaneously achievable rate pairs, assuming a general K-user perturbative channel model using genie-aided techniques. Furthermore, an achievable rate region is obtained by time-sharing between certain single-user strategies, and it is shown that the latter can achieve better rate tuples compared to treating nonlinear interference as noise.