OpenSees, and nonlinear structural analysis in general, is a simple endeavor. It is not complicated. It does not need to be complex.
Although this clip from Bull Durham, the best baseball movie of all time, refers to throwing, hitting, and catching the ball, the simplicity translates to nonlinear structural analysis.
You build the model. You analyze the model. You post-process the model.
You form the equations. You solve the equations. You update the equations.