Omnibus F4 pro UART

I have flashed my Omnibus F4 pro v3 to Ardupilot and would like to use OrangePi Zero as an companion computer.
However, I cant set up any telemetry link between omnibus and orangepi.
I use UART1 as telemetry output on Omnibus and have enabled UART1 on Armbian running on Orangepi.
The biggest secret is the uart voltage level on Omnibus, is it 5 or 3.3V
On OrangePi it’s 3.3V

Almost all flight controllers use 3.3v. including the omnibus f4 pro.

Yes the Omnibus UART is 3.3v as are all other signal pins on the Omnibus F4 pro v3.
I use two of them but interface directly with a Raspberry Pi 3B and Zero W tx/rx pins directly, that operate at 3.3v signal levels.