Well you spoke to the village idiot at Oppo.
Your sub has one XLR input for connection. Sub outputs are mono. The left and right sub outs carry the same signal. Some subs have left and right inputs, and have buffer amps in case you want to send a full range left and right signal from a preamp or in your case your Oppo player. If there is only one input buffering by the sub is not possible.
I have never seen buffered outputs on any unit.
That means that if you connect right and left outputs other than sub outputs which are already mono, then you will convert your device, in your case the Oppo player from a stereo unit to a mono one.
So to do what you want to do requires a
buffer amp like this which can be converted to a summing amp, by an easy internal mod. I know I have one.
In the unlikely event the right and left XLR outputs on the Oppo are buffered you can easily find out.
After you combine the left and right XLR outputs of your Oppo and send them to the single LFE output of your sub just play a signal that has left sided content only. If you only hear sound from the left speaker then those outputs from the Oppo are buffered. If on the other hand you here sound from the left and right speaker then you have mono because the outputs are not buffered. In that case you will need a buffer/combing amp between your Oppo and the sub.
Don't bother asking Oppo as for almost 100% certain the person you talk to will not know what buffering is and the requirement for it.