OLED brightness on linux has been hit & miss for the past 4 years or so (since OLED came to laptops).
There are numerous ways to set OLED brightness manually, here's the ones I found more useful:
xrandr --output $oled_screen --brightness $percent
$percent is a value between 0.0 and 1.0, $oled_screen is your oled panel screen name. To get a list of attached screens run xrandr without any arguments.
redshift -m randr:screen=$oled_screen -P -O $temperature -b $percent
This is an example of how to manually set brightness and night light filter. $oled_screen is the same as above, $temperature is the color temperature of the display (6500 for normal, unaltered colors, 3500 is a usual value for a night filter), $percent is a value between 0.1 and 1.0 and it specifies the brightness of the display. It acts the same as when using xrandr, but redshift prevents you from setting values lower than 0.1 while xrandr allows even values of 0.0 (pitch black)
redshift -P -O $temperature -b $percent
Same as above, but changes will be applied to all screens. Useful if you don't want to look for screen names when you only have 1 screen anyway.
Anyway, I doubt you are just looking for "how to set brightness using a command" and would appreciate a solution that "just works" on your shiny amazing OLED display. This repsository is probably the solution you are looking for. It "just works" in most cases, and with little tweaking you can get the brightness control similar to how it works out of the box on LCD displays. There is also a comprehensive documentation in README.md