In optical burst switching networks, contention resolution schemes as well as contention avoidance schemes reduce the burst loss probability. These schemes delay the burst delivery and may change the burst arrival sequence. In this paper we present an analytic burst reordering model and derive analytically the impact of a time-based burst assembly scheme on the burst reordering pattern. Our results hold for a general network delay distribution. We apply the IETF WG IPPM reordering metrics and calculate explicitly three reordering metrics assuming a general burst delay distribution: the reordering degree, the extent metric for buffer dimensioning and the TCP relevant metric for TCP throughput estimation. We show that our analytic model represents a worst case reordering scenario, which enables studies on the upper layer protocol performance in OBS networks without excessive multi-layer simulations.