Yes, the server listens for PXE requests on the same port (“It’s Complicated”).
If you enable the (deprecated) web interface for the OSD server, there are configuration options to allow DHCP and OSD to coexist on the box (without Option 43 configured, the client sends a second DHCP broadcast to discover a PXE server, and when nothing responds the client assumes the same address as the DHCP server). I don’t know whether there is a Fixlet/Task to do the same.
I think generally it’s recommended to put DHCP on another machine but it can be made to work if they’re on the same machine.
The longer version is, that there are a lot of different ways to configure the environment. In the simplest form, single-subnet, your PXE client boots and broadcasts to find a DHCP address and is serviced by the DHCP server. After it gets an address, it broadcasts again to find a PXE server. In a single-subnet, REMBO hears this second request and services the client.
In a multi-subnet environment, you might have DHCP servers on each subnet (often serviced by the routers), or you can configure the routers to forward the DHCPDiscover broadcast packets to the DHCP server (Cisco calls this “ip helper-address X.X.X.X”). You could configure the routers to also forward DHCP broadcasts to the OSD server and Rembo would service PXE via broadcast discovery.
What’s more commonly done is for the router to only forward the broadcasts to the DHCP server, and configure DHCP options on the server for PXE discovery (Options…something and Option 43). When the client sees these options, it is informed of the PXE server’s address and any PXE menu options. Then the client can contact the OSD/Rembo server directly without further DHCP broadcasting required.