Why SACD Disc playback sounds better than its equivalent ripped DSF file / streaming

The worst place to store the music file to play is a external hard disk connected (and power supplied) via USB: it is very noisy (several times tested by myself and many, many times read in forums)

The other point to consider is that USB connections are more difficult to manage by the manufacturers than the "internal" connections, that I guess in your Oppo is a I2S.

I do not know if possible, but using an external computer to "play" the file with a good app (Roon, Audirvana, ....) and feed the Oppo as an external DAC, would make a better comparison.

https://hydrogenaud.io/index.php/topic,124798.0.html

It appears that our observation SACD Disc playback sounds better than its equivalent DSF file playback has been observed by others as well.

Try the same experiment but with a redbook CD. 
The outcome will most likely be the same. 
The reson, most likely, is the computer feeding the signal into the Oppo via USB is too noisy and the Oppo is not able to cope with amounts of jitter in the data stream. 
The quality of the USB cable matter as well. 
 

I compared CDs played on a marantz sa-10 to the exact same albums ripped as FLAC fed into the sa-10 via USB from the Aurender N200. It sounded identical. 

Well it seems cd are pretty good with quality sound you hear that ,audiophiles....