The paper is devoted to promoting and describing the use and advantages of a software package COOPT for optimal control of large scale differential algebraic equation systems, including also the nonlinear case. It is emphasized that the package uses the multiple shooting discretization method, which is also briefly described. Next the basic structure and implementation of COOPT is described with particular emphasis given to control parametrization, optimization constraints, control continuity and other constraints, residual computations, integration tolerances and finally to sensitivity of optimal solutions. At the end, a simple example is provided.