Blu-ray performance would be superior, if only slightly. The PS3 doesn't de-interlace 1080i material at all, while the superior de-interlacing of the Oppo 83 would handle that, and thus give you a better result.
If you have a 60gig PS3, chances are it's also rather noisy. The Oppo isn't. I've used my PS3 for BD before I got my Panasonic, and later my Oppo, and one of the bigger improvements I got in practice was that the noise level of the player went down considerably.
But it's with DVD playback you'll see the largest improvement for sure. 1080p output from a Blu-ray Disc with 1080p material on it is not difficult or expensive, and it is something that most players do almost perfectly. Don't buy an Oppo BDP-83 and expect to see some huge improvement in general BD playback, because there isn't one to be found.