Hello Chris,
Have you considered using flac?
It is supported by Decent Sampler right now, does not degrade sound at all and can do wonders regarding file size. It generally divides the size by at least 2.
While ogg, opus or any lossy format achieved in the frequency domain may seem appealing regarding file size, they are not particularly well suited for virtual instruments where you need to randomly access little parts of a sample.
This kind of codec introduces phase shiftings thus you don't get an exact match in time compared to time domain codecs.
Apart from the hdd saving, they probably won't save any ram because they would probably have to be pre-buffered in RAM to be played correctly.
Hardwares using lossy sample compression (typically arcade games or game consoles in the 90s or the early 2000s) tend to use ADPCM variants (achieving an approximate compression ratio of 4:1) because it is very cheap to compute, so it can be streamed at he same time it uncompresses. Moreover given its time domain nature, it is an exact match in time compared to the original audio source so it is suitable for precise sample loops.
All this to say, even if Decent Sampler would support ogg or its most recent iteration (opus) there are more options to consider before committing to a lossy format.