Something like this should work for you.
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FYI, the "audio only" output on the Oppo is not actually audio only. It's a full audio/video signal but it's raw unprocessed video. The "Audio/Video" output on the Oppo is the video signal that adds all the video processing which you can configure in the Oppo menus (color space, noise reduction, color/hue adjustments, etc.).
The only device that I think might actually separate out the audio stream from HDMI is a Lumagen Radiance video processor. I have not tried this, but Radiance has two outputs and you can configure one for "audio only" and the other for "video only".
You could try that Orei splitter that cleeds suggested. Not all HDMI splitters are equal. If this doesn't work, you might try a Gefen Toolbox splitter (part # Gefen GTB-HD4K2K-142C-BLK ). They are very expensive, but are engineered better to support different HDMI protocols/devices.
But it works...
All you need is a i2s DAC. Plenty out there.
I think it answers the OP’s question.
You sir, are quite free to offer higher quality solutions.
It works fine with this guys 205.
The device @golfnutz identified might be exactly what you are after; I'm not sure about your objective, but the audio return channel (ARC) on my TV system allows me to send a 4k signal from an Amazon Fire Cube to the TV and using an additional hdmi cable from the TV input and output panel, I get audio back into the pre-pro through the ARC channel. (The pre-pro doesn't process 4K video, thus, the work around).