The PeV energy photons from the Cygnus bubble recently measured by LHAASO suggest that star clusters are among the population of sources called PeVatrons. In this work, we examine the capability of acceleration at the wind termination shock of Cygnus OB2. We find that spatially dependent Bohm diffusion downstream of the shock is favored to explain the observed gamma-ray spectrum, and that Galactic cosmic rays that have penetrated into the bubble contribute a significant fraction of the gamma-ray flux above $\sim$ 300 TeV. We also find that slow diffusion of nonthermal particles in a region extending to at least 150 pc from the center of the bubble is necessary to reproduce the measured gamma-ray morphologies. Our findings enable new insights into the physics of particle transport near astrophysical sources.