Let's see if I understood you correctly: you are in fact wondering which DAC is better, the Oppo BR player, or your receiver's DAC?
If you connect your BR player via analogue cables, it means that the player does the conversion. Via HDMI, you have the option to let the receiver do the conversion; you decide which one sounds better.
I wouldn't say the player you mentioned would be useless; if the DAC in the BR player goes to 192 kHz, you would be able to listen to DVD-A (for example) without any other DAC involved. Straight to an amplifier.
If you're talking strictly movies, than yes,kinda useless. Movies are encoded at 48 kHz usually, so unless the player has a super DAC, you won't hear any differences.
What type of receiver you have?
M80 v3, 2 x Emotiva UPA-1 Monoblocks, XPA-5, UMC-1 PrePro
Pro-Ject RPM Genie, Carver C2 Phono Preamp