Extract DSD from Oppo BDP105 to RME ADI2 Pro FSR?

Well it is possible to get raw DSD output from an Oppo, using DSD-over-HDMI, in conjunction with a certain type of HDMI de-embedder device ...
but this will give you an LVDS-I2S signal, and unfortunately the RME does not have such an input, unlike its Topping and SMSL competitors.
I'd have to use USB to play back DSD, but I'd be ok with 96k out. I2S would be better, but most DACs with I2S are typically ESS based for anything reasonable priced. I dont like the ESS sound signature, as its on the brittle side up top. Playing native DAD discs over the 96k PCM out to thr RME is magical, so anything more than 96k is unnecessary for me.
I'd have to use USB to play back DSD
Yes ... and I don't see any way of converting a DSD-I2S stream to USB-audio protocol.

May I suggest you use your OPPO to do SACD ripping, and use a computer to it play back through your RME?
Yes, great idea. If you wish to include your RME DAC at the heart of your music playback system, it makes sense to make the transition from optical disc playback to audio file playback.