use the line from the 2nd LNB port of the Sky dish. You might use the Loop-port of your Openbox to connect it to the 2nd LNB port of your digibox (at least you can try, depending on the way your Openbox acts on receiving LNB power supply on the Loop-port, but most likly Loop will only be active when Openbox is in Standby mode)
I had a Sky+HD digibox as well (6-7 years ago) and if Sky has not changed the software of the receiver (most likely not *g*) with default configuration of the digibox you will encounter problems if you want to keep the box running:
unfortunately the Sky digibox do not prefer one LNB-input port (like most receivers try to use LNB-1 if possible). So if LNB-port 2 is not connected to the digibox (or not active because the Openbox is running itself and does not loop the signal) you might be successful on switching channels, but you might also end up with "no signal"
So if you want to keep original BSkyB-receiver active you need to disable 2nd LNB input in the configuration menu. At least some years ago this was a hidden menu of the digibox which requires to press blind a number of buttons to get into it and where you can switch to single LNB mode/only one active tuner). Use Google and search for digibox hidden menu (or something like that). At least some years ago with only one LNB-port connected/configured as active on the digibox you will loose features like Sky Anytime (if this Sky service is still in place, I am not sure *g*)