How to display voltage on OLED?
I don't know how to drive OLED screen

So look it up.  Read the datasheet to see what commands it has, then program the microcontroller to emit those commands as needed.  This is what anyone else here would need to do too when faced with a new display they haven't previously used.

Considering that the display is specified as "128x32", the commands probably only give you ways of lighting pixels.  That means you have to do the character generation yourself if you want to display characters.

From the level of knowledge implied by your question, the above is over your head.  Go buy a meter.  It seems you want something called a panel meter.  Those sense a voltage and drive a display, but are intended to be integrated into bigger devices.  These things come in many flavors, including backlit or not, LCD, LED, different character sizes, etc.