OS: WIN 10
AMD Driver: 18.9.3 (But I've tested 5 different drivers dating back to launch day for RX Vega)
GPU: RX Vega 64 LC
RAM: 16gb DDR3 1600mhz
Several months ago I played Oblivion and Super Sampling was working like it has for a long time, long long ago there were some drivers where it didn't work but for at least 2 years it has worked including since I've had RX Vega 64 but given that I tested the old drivers including the ones I remember using around the time I was playing I suspect it's a Windows update that's broken it.
In my testing neither MSAA or SSAA override works, changing game settings such as disabling HDR doesn't make a difference.