Burst assembly plays an important role in reducing the end-to-end delay of packets transported through optical burst switching (OBS) networks. Several methods have been proposed to reduce the delay of the packets buffered at ingress OBS nodes. However, these have created significant estimation errors, which result in wasting the reserved bandwidth or increasing the delay of excess packets. In this paper, we propose a model of optimal burst assembly for delay reduction, which minimizes the estimation error, eliminates the excess packets, and decreases the blocking probability of scheduling.