Not sure what model of tv you have but are you actually looking to source audio from the tv directly? That is, it is a smart tv or you have over-the-air content in the tv you want to get to your soundbar? That is all ARC would provide and since it is the same audio quality as a digital optical or coax, you don't necessarily need ARC.
I don't see that the soundbar is capable of hdmi input at all, let alone ARC, so any content from the tv to the soundbar would be by one of the other input options (appears to be capable of digital coax or optical or analog inputs only). What other audio output options does your tv have other than ARC?
I'd probably just connect your cable box to the Oppo and use one of the hdmi outputs to the tv for best video, and then connect the Oppo to the soundbar by one of the available digital/analog inputs.