Once the air is at a sufficient level, the indicator "Vehicle is not ready", the largest red light on the instrument panel, turns off.
You can switch off the engine again now. Observe the error light and the battery indicator light. The battery indicator light indicates that the power supply of the bus is the battery.
Now once turn on the high beam (foot switch at the bottom left or [F] button) and take the opportunity to have a look at the blue indicator light "High beam" at the panel. The so-called beam flash, the very short flash of high beams, also in OMSI grants oncoming vehicles illegally right of way. Remember this to avoid accidents, if you're driving in public transport.
The indicator light "Central lubrication active" (orange indicator light on the right of the large error light) and coolant level critical (indicator light on the left of the large fault light) are initially without meaning to you.