In oxide dispersion strengthened steels the interactions between dispersoids and dislocations determine the material's plasticity. Using three-dimensional Discrete Dislocation Dynamics simulations, the effect of Y2O3 dispersoids on the motion of dislocations in BCC single crystal PM2000, a commercial alloy candidate for gas-cooled reactors, has been studied. The dispersoid distribution used in this model has been derived from experimental TEM observations of PM2000. As screw dislocations are predominant in the studied material, the behaviour of a single screw dislocation under shear loading in a distribution of spherical Y2O3 dispersoids is studied. The critical resolved shear stress, the minimum value of the external stress needed to move the dislocation through the obstacle field, is found to be only slightly lower than the experimentally determined value revealing that dispersoids are the main hardening contributions.