There are 2 ways to go here:
1) get a Final Drive transformer buffer to change from bal to SE:
2) add an external DAC with SE outputs.
This will sound better, PROVIDING that you reclock the Oppo digital output and use a really good coax cable. Here is a jitter plot to show how much jitter before and after the Synchro-Mesh reclocker:
Here is the jitter added by different coax cables:
The only thing that matters when using a CD player as a transport is the jitter. The Sychro-Mesh and a good BNC cable will cost ~$1025
Most decent DACs will beat the D/A in the Oppo, but you will need to spend $2-3K for new or $1-2K used. The best DAC will still sound mediocre without a low jitter source.