A generalization is an assumption made on the basis of only a fraction of the sources provided, while an oversimplification is the process of reducing the quantity of possible causes for an event.
A generalization differs from an oversimplification in the number of sources they base their assumptions on, as an oversimplification is often based on a single or too few causes, whereas a generalization - while the number of causes is also reduced - the sources are originally many, and the majority are unaccounted for.
Based on this information, a sound generalization differs from an oversimplification because it not all of the sources are taken into consideration.