@auxinput is correct and the HDMI input on your DAC has nothing to do with the HDMI output on your Oppo. They are two different animals. They are the same physically but the type of signals they require are completely different. Do not plug one into the other.
If you want to use the HDMI input on your DAC you need the Oppo I2s card, no ifs or buts or whatevers.
The HDMI output on your Oppo transmits Video and Sound for watching movies and as Auxinput correctly said if you decode the sound from this HDMI cable it won't be better than COAX.