@Michal-Dobrzynski Please do not forget to remove the pin3-pin4 short circuit if you want to plug the Oled Expansion to any genuine Onion Omega Expansion Board Header directly because Omega2(+) may be damaged if you set GPIO11 to Low.
There are I2C level shifters on the OLED Expansion.
Pin4 of the Expansion Header provides the voltage to the lower voltage section for those level shifters.
But pin4 of the Expansion Header is connected to GPIO11 on the Omega2(+) board.
So @Onion must set GPIO11 to High (~3.3V). It's a hidden simple hack that @Onion forgot to mention for example here.
You could find some further technical details here.