NHD-0420DZW-AB5 OLED Display incorrect output

Hardware
2

So you object instantiation looks like this?

Adafruit_CharacterOLED lcd(OLED_V2, D7, D6, D5, D4, D3, D2, D1);

If so and it still doesn't work try putting RS to D0 - maybe the LED on D7 keeps the device reset.
Also make sure the display does work with 3.3V logic when powered off Vin (5V).

BTW, there was a similar thread not long ago