This paper addresses a continuous-time preview control problem in output feedback setting. By a state transformation approach, it is shown that the optimal output feedback law is implemented in two different ways. The first and second implementations have the structures similar to the Smith and observer-based predictors for input-delay systems, respectively. In order to illustrate the features of the optimal control law, we design an Load Frequency Control system, which utilises load demand prediction. Although the accompanying prediction error obstacles the performance improvement, it is demonstrated that the adverse effect is mitigated by the weighting function technique.