This paper carries out an inter-area oscillation study using the accurate model of the Greek power system. Taking into account the complexity of the system under examination, an automatic toolbox is developed that can define the oscillation modes and provide a graphical representation in order to detect if there exist any area separation in the selected frequency. As case study, an inter-area oscillation between Greece and Bulgaria was simulated. The results showcase that the toolbox can effectively define the frequency, amplitude, damping and modes of the system and detect if there exist any significant phase angle difference between the buses, which is an important indicator of the stress in the power system. This information can be used to determine the critical points for employing wide area monitoring and damping control systems in order to damp oscillations and improve the overall reliability of the power system.