Most subs are designed/marketed for home theater use, so confusion can come up when used in stereo (2.1) systems.
Undoubtedly the 7.1 outputs on the Oppo are meant to connect with an A/V preamp/receiver and so have no way of controlling subwoofer volume. So not recommended for use in a 2.1 system.
So that leaves you connecting the sub via the preamp. If the preamp has two pairs of variable (volume controlled) outputs one pair can run to the power amp and the other to the sub, easy-pleasy. If the preamp only has one pair of outputs you can connect "Y" cables to the preamp output, with one set for the left and right inputs to the power amp and the other set to the sub.
Note that if you want to save money on the cabling to the sub, you should be able to verify if one low level (RCA) input has been designated for a single channel input (either labeled on the sub or stated in the owner's manual). If you do want to run a single cable to the sub, just use a "Y" cable to combine left and right channels from the preamp.
An alternative (if the sub has speaker cable inputs/outputs) is to run speaker cables from the power amp to the sub and then more speaker cables to the left/right speakers. In this way the low frequency signals would be filtered away from the left/right speakers.