MakeMKV is not a converter, it extracts the movie from the disc structure and copies it into a mkv container, it is the exact same video/audio on the disc. As such any player must be capable of decoding Blu-ray video streams in mkv for that to work, I don't know of the Oppo is but from the sounds of it, it isn't & Oppo's are not very good digital media players they are primarily disc players.
Make sure the firmware is up to date on the Oppo 93 in case they made any improvements in this area that might unlock playback of the files from MakeMKV.
The web page for Oppo states it supports mkv, probably H.264 video at least. So run the file generated by MakeMKV through Handbrake (H.264 encoder) using the high profile preset and that will probably give you usable video on the Oppo. If you want to retain the original audio use the mkv container in Handbrake and on the audio tab set it to AC3 or DTS pass-through depending upon whats in the original mkv file.