In this paper the performance of a hybrid optical network composed of multiple Ethernet Passive Optical Networks (EPONs) at the access connected to an Optical Burst Switching (OBS) core is evaluated. The EPONs make use of the IPACT algorithm to manage the upstream transmissions, whereas the OBS edge node employs a time-based assembly scheme for burst aggregation. This leads to interoperability issues between access and core which are investigated in this paper. The hybrid network scalability is proved by assessing the transport layer performance with different numbers of EPONs served by the same burst assembler and different numbers of Optical Network Units (ONUs) per EPON. The impact of the main design parameters affecting the interaction between EPONs and OBS is estimated. The transport layer performance is evaluated considering heterogeneous traffic flows by simulating both TCP and UDP sources.