ONED90 is a computer program to simulate the nuclear core transient in terms of operational transient. It simulates mainly xenon transient which occurs with time step of 10 to 20 minutes during 3 to 4 days. To predict core state with accuracy, the power adaptation method, control rod adaptation method are employed. After tuning initial core condition ONED90 predicts core state with great accuracy. This report describes the Nodal Expansion Method, adjoint flux calculation method, equilibrium xenon concentration calculation method to improve accuracy, and Graphic User Interface for ease of end user. This report also mentions about the verification of ONED90 against the steady and transient core state. (Author).