I think @rdgrimes has a clue here, the Oppo 93 is a 2012-ish model and may have compatibility issues. Click the link for help.Thanks rdgrimes and Chromejob for your fast reply.
I've Nvidia Sheild TV with HDMI and it works fine with Yamaha 3080 video and audio plays. When playing with Oppo BDP 93 the any audio files plays but there is no Audio (sound).
This page might provide some help, too. Make sure that HDMI Audio is not set to OFF. Based that page's info, I'd try Auto, then switch to Bitstream if that doesn't work.
As I suggested, find some digital media files that don't play from the Oppo (according to the manual, you can play from a USB or e-SATA drive), then try them via the AVR's USB drive.
Link please?eARC via firmware update confirmed for 2018 Yamaha AVRs. You can check it on the Yamaha website.