Object-oriented programming is an alternative to overcome the problems associated with the development, maintenance and update of large software by electrical utilities. However many applications in the electrical industry depend critically on the computational efficiency of the implementation. This paper applies the object-oriented approach to the problem of power system dynamics simulation. A decomposition of the electric power system into classes, suitable to include all elements required in this problem, is presented. A structure for linear systems solution aiming at computational efficiency for large-scale systems applications, is proposed. Tests using four equivalents of the South-Southeastern Brazilian interconnected power system are carried out. The performance of the object-oriented program, developed in C++, is assessed in several computers. Real-time simulation was achieved for medium-sized power systems in lower cost high performance computers. The implementation is compared with an industry program written in Fortran and similar performances are achieved.