What you have done is switched where the deinterlacing is being performed as well as saved conversion to and back from analog which may be the majority of the picture improvement. Not sure if your Sony DVD player outputs 480P, but you can test who you thinks does the best job of deinterlacing by having it done in the player, AVR, and your Sony display and see which yeilds the best picture (all with HDMI else it is not an apples to apples comparison). You can do the sme and add scaling when you get the Oppo (essentially comparing the Oppo to the Sony display).
This article on CNET may be helpful:
Upconverting HDMI DVD players: Fact vs. fiction