Optical burst switching (OBS) is considered as an efficient switching technique for building the next generation optical Internet. An offset-time based scheme has recently been proposed in order to provide quality-of-service (QoS) in OBS networks. Unfortunately, the proposed service differentiation has several problems. The aim of this paper is to address these problems and introduce the concept of proportional QoS into this OBS paradigm. An intentional dropping scheme is proposed so as to give a controllable burst loss probability for different service classes. In order to achieve flexible packet delay differentiation, we extend the well-known waited-time-priority (WTP) scheduler to form a burst assembling scheme. Simulations are conducted to evaluate the performance of our proportional QoS provisioning within OBS networks in terms of burst loss probability and packet delay.