Oil induced instability, is a frequently encountered phenomenon causing system instability for rotors supported by hydrodynamic journal-bearings. In this paper a flexible rotor, simply supported at one end and with oil lubricated journal-bearing at the other, is analytically modelled. The rotor system is modelled in two ways namely as a discrete system by finite element method (FEM) with nonlinear journal-bearing and as a lumped inertia system with linear journal-bearing. The analysed rotor-bearing system is a Bently Nevada Rotor Kit Model RK4 with Oil whirl/whip option. Results obtained from the simulation of the discrete rotor model with a nonlinear journal-bearing indicate at which rotational speed the oil induced instability (oil whirl) will occur. Campbell diagrams are shown for the lumped inertia rotor model with linear journal-bearing and the critical speeds are predicted. From the results the accuracy of the analytical speed-dependent bearing coefficients are evaluated. These coefficients were derived from the nonlinear bearing impedance descriptions by D. Childs. The bearing impedance descriptions method is a method valid for all L/D (length to diameter) ratios, and all journal eccentricities. The simulation time is significantly reduced by using a lumped inertia rotor model with linear journal-bearing. Critical speed obtained from Campbell diagram predicts a threshold speed of instability which is about 0.35% higher than that predicted by the discrete rotor model with a nonlinear journal-bearing. Compared with results collected from experiment, the simulation results predict a threshold speed of instability which is about 5.69% higher (linear analysis), or 5.36% higher (nonlinear analysis).