If your TV only has an optical audio out then you will need a digital to analog converter that would connect to any input on your stereo except phono with standrard audio cables and to your TV with an Toslink optical digital audio cable.
If you have a set top box (cable. sat. fios. etc.) then you can connect this to your stereo with the standard left/right audio cables. Same for a disc player. Video would connect to the TV with HDMI. You might need to set a menu in each source to get them to work properly this way.
the onkyo should have a DAC in it. like the audiophile above sed, connect the 2 together with SPDIF or analogue, id stick with the SPDIF at this point.
most TVs do have DACs for their audio outs, but the DAC on the onkyo should be better.