The optical cable will be limited to lossy DD 5.1. The Oppo is a universal player supporting Dolby TrueHD, DTS HD Master Audio, DSD and multichannel PCM. All of them exceed the bandwidth available using an ARC or optical connection. So, the HDMI audio connection to the receiver will be necessary to have access to any of them.
The Apple TV 4K converts everything to LPCM by default. Again, bandwidth issues will prevent any multichannel PCM signals going over an ARC or optical connection. So, the output format will need to be changed to Dolby Digital 5.1.
To set up optical output on LG TV, bring up Settings>All Settings>Sound>Sound Out>Use Wired Speaker>Optical Out Device. Then, go back to Sound and select Advanced Settings>Select HDMI Input Audio Format. Select Bitstream for every connected port. Then, go back and scroll down to Digital Sound Output and select Pass Through. This will allow the DD 5.1 signal to be output without a conversion by the TV to PCM 2.0 to the receiver.
You are right, though. While the LG OLED TVs put up amazing images, the settings really suck and aren’t for the faint of heart.