In slotted systems, the traffic aggregation strategy heavily impacts efficiency and becomes even more significant in the environment of OBS architectures. One may choose to wait until a slot is filled up or to set a limit to the assembly time, so as to offer delay guarantees to QoS demanding traffic at the cost of inefficiency. When both real-time and elastic traffic must be supported, their contradicting requirements present a challenge on the slot assembly strategy. In this paper, such a slot aggregation strategy combining high utilization gain with service guarantees is presented and evaluated