Just to clarify, by ethernet port you mean one of the LAN ports, correct? Both the LAN and WAN ports are ethernet ports
. But semantics aside, you'd want to connect it to the WAN port (the one with the flower looking thing on top). AirPort Base Stations automatically switch that port to work with whichever setup you use in AirPort Utility, which is nice as it leaves all 3 LAN ports open for other devices to join your network.
A piece of advice, enable the g signal on your modem/router and make the AEBS Wireless N only (5 GHz). This way you'll have a dual band network where b/g devices can join the O2 network and N devices join the AEBS network. It's still the same network since they're on the same subnet (albeit with different SSIDs), and the b/g clients won't slow down any N clients.