I dont use any proper interface. I just use add a "browser" window in OBS and load the Main Display using 192.168.1.1:4316/main.
I use a #0000FF (blue) background in OpenLP, then apply the Colour Key effect in OBS to give a transparent background.
I've found it works flawlessly.
I run both OBS and OpenLP on the same machine, but it would work just as well on different machines.