OM Series devices include two ethernet ports. One port can be used as the WAN interface, while the second ethernet port can be used for any LAN devices such as a switch, VOIP phone, computer or another OM Series access point.
There is no designated WAN and LAN port. Both ports will act as both WAN and LAN. The firmware on the unit is able to determine which is which.
You can daisy chain the OM2P node to another OM2P node via an ethernet cable so long as you're running ng-r397 firmware or later.