The paper deals with two examples, which show the efficiency of OO modelling approach in control engineering. The first example - an optimization of three tank hydraulic control system shows the efficiency of the OO modelling approach in control engineering education. A very efficient environment is the combination of Matlab Simulink and a Modelica based OO environment Dymola. Two different control strategies -- PID and cascade control are demonstrated. The second example shows the efficiency of OO modelling approach in an industrial application. DYMOLA-Modelica was efficiently used for studying a pendulum system in a stone wool production technology in a Slovenian company. In this system linear (horizontal) movement of a bottom edge of the pendulum system is a very important technological feature in forming the secondary wool layer. The simulation results showed that it is possible to achieve more appropriate movements with regard to position and velocity.