The OCL language is now well-accepted by the modeling community. To be compliant with the regularly updated standard, evaluators must ensure that OCL is a strongly typed language. Indeed, during the analysis of a rule, the evaluator must ensure a correct and a deterministic type of each sub-expression handled. We have chosen to provide a dynamic type system for our OCL evaluator. Moreover, formalizing the OCL type system has the advantage of helping to clarify the standard. This formalization uses the natural deduction logic to represent typing rules and was implemented throughout a type synthesizer in a platform named Neptune.