Requirements specification is a collaborative activity that involves several developers specifying the requirements elicited through several stakeholders. Operation-base merging allows combining specifications using the information available about their state as well as their evolution or change. Thus, leading to a more precise, accurate and efficient merging. Differencing specifications is a tedious, complicated, and a crucial process needed for operation-based merging of specifications resulting from collaboration. An approach for differencing Object-Oriented formal specifications is proposed in this paper. The difference is modeled as a set of primitive operations and is produced based on the matching results of specifications' elements. These matchings are calculated based on an approach employing elements' syntactic and structural similarities. The proposed differencing approach is empirically validated.