I agree you could mod the OPPO and that might serve you well for awhile but think there are some good reasons for a separate DAC.
Transports in disc players fail, they always do eventually. My experience has been that as player models change so do the transports inside them and it's sometimes impossible to find a replacement for a model that's no longer current. Without a working transport the DAC inside may be useless, no matter how good it is.
And my experience is that most of the advancement in audio quality over the last few years has come from the improvement in DACs. You could reasonably argue that speakers and amps have improved, as well, but the degree of improvement in DACs seems to me to be greatest. I expect that will continue.
And, an external DAC gives you more flexibility as new sources of music come along.