The new O3 Air Unit works for me on 4.3.0 with:
OSD_TYPE = 5
SERIALn_PROTOCOL = 42 (n corresponds to the UART where the Air Unit is connected)
MSP_OPTIONS = 4 (uses Betaflight fonts; without it many symbols are missing)
I also had the following defaults, didn’t change them:
SERIALn_BAUD = 115
SERIALn_OPTIONS = 0