ARC requires both the TV and audio system to support it.
The Onkyo does have an optical audio input. You will use that to connect the TV to the HTS. No loss of quality just requires an additional toslink cable. You may need to turn the optical output on, the TV speakers off, and change the output to bitstream in the TV menu to get it working. You may also have to assign the optical connection to an input name in the input set up menu of the Onkyo.