This paper presents an open shop scheduling problem based on a mechanical workshop, whose objective is to find the sequence of operations that minimizes the total flow time considering release dates (Om|ri|ΣFi). For that reason, we first formulate the problem as a mixed integer linear programming model which applies the classic scheduling constraints but also adds different resource constraints related to the human competences and their availability according to their mastered skills in order to execute one activity; thus, the model is presented and solved optimally. Besides that, a genetic algorithm is proposed to minimize the computational time. To provide a clear perspective of the method and its advantages with respect to the exact method: computational experiments using an approach of the literature are presented. The different numerical instances (different configurations of setup and processing times as well as release dates) are tested, showing good solutions in an acceptable computational time.