For flac, wav, ape etc.
"Options > Media Network > {select your DLNA Server} > Audio > Mode" set to "Specified output format".
"Options > Media Network > {select your DLNA Server} > Audio > Format" set to "PCM L16 No header", or "PCM L24 No header" if that works.
"Options > Media Network > {select your DLNA Server} > Audio > Advanced > DSP Studio > Output Format" set all Input formats to Output 768KHz.
For Bitstreaming DSD over PCM (DoP)
"Options > Media Network > {select your DLNA Server} > Advanced > Bitstream DSD (requires DoPE compliant renderer)".
Note: DoPE is DSD over PCM Ethernet.
The above should work as you wish. But I haven't tested, and can't, as I don't have an Oppo UDP-203. A search of the forum will find discussions about what the Oppo 203 can do, and can't. It is a fairly common topic.
For example:
https://yabb.jriver.com/interact/index.php/topic,117958.0.htmlBut the Oppo 203 may not support DoP over DLNA, but instead support Native DSD, in which case you will need to tweak the configuration.
Note also: What the Oppo 203 can do over USB is different to what it can do over DLNA.