I haven't heard the Oppo, so no opinion on it.
I owned the Rega Apollo and searched for a while to find a $1k and under upgrade to it. I bought a Rega DAC and have no regrets whatsoever. Even though they're the same price, the DAC is in no way a lateral move. Everything the Apollo did, the DAC does quite a bit better. The DAC is on the level of the Saturn, not the Apollo. Many have said the DAC is different that the Saturn, but not better or worse. I haven't heard them side by side, but I've heard the Saturn more than enough to know the DAC is on equal footing with it.
If you liked what the Apollo did in your system, the DAC may very well be the answer. It was to me. Nothing beats a demo, especially in your home. If you have a local Rega dealer, you should make it a point to hear one. If not, I'm pretty sure Audio Advisor and Music Direct have a 30 day money back policy.