Consider as a general rule the 5100 defines the speaker output, via the setup, along with the Amp(s) and Speakers connected to it, and the DSP mode selected.
The Oppo has a number of output connections
1. RCA/XLR stereo ouptuts, usually intended for Stereo reproduction of CDs using the the Oppo to decode
2. Coax/Spdif, usually intended for Films (Dvd, Blu-rays, etc) however this output is limited to a compressed 5.1 Dolby or DTS formats, or stereo PCM.
3. Pre-outs, usually intended for AVRs/Processors which cannot handle the hi-res audio formats however this output would be limited to 7.1. Atmos/Dts:X object data would be discarded and no overhead speakers would be supported.
Lastly
4. HDMI, this will provide the entire range of formats supported by the Oppo and discs decodable by the 5100
The Oppo has two Hdmi outputs. One is intended for the 5100 to passthru the video to the TV or projector. The other is audio only where the direct connection to the TV or projector is required and the audio is decoded by the 5100.
Unless you plan to connect the Oppo to a separate Stereo amp for CDs then HDMI is the only connection required. There is probably no real benefit using the RCA/XLR and the Oppo to decode CDs just to have them pass through another set of AD/DA converters in the 5100