The rules state an orbital strike relay is a ranged weapon that can only be used if the model did not move in the previous movement phase, yada yada yada.

I always took that as meaning the OSR is just like a heavy weapon, although it does not benefit from relentless. And afaik, the BBB is quite specific in that you are only allowed to use one weapon a turn/phase unless it is specifically allowed by a special rule to fire more than one (ie, Sister's Seriphums).

Since OSR does not say specifically that it can be fired in addition to the model's normal ranged weapon, I would not think you are allowed to fire both in the same round, as cool as that would be.