For automatic buses:
Gear lever: 4th gear, parking brake on; 3rd gear, parking brake off.
Red buttons, left to right: drives light, first, second, and third passenger lights (just like F9, F10, F11, and F12, same order).
Black buttons, starting at the top going counter-clockwise: engine, reverse gear, neutral gear, drive/forward gear.
I have the D-Pad programmed in the Logitech Gaming Software program because OMSI does not recognize it. Up, hazard lights (or four-ways as others would call it); left, parking/very dim headlights; down, flashers/high beams; right, normal headlights.
On the steering wheel, left side buttons going down: turn driver's head left, turn signal off, electrics; right side buttons going down: turn driver's head right, and horn. Didn't find a useful bus or switch for the bottom, so I programmed it as the red information line that appears at the top of the screen.
Flappy paddles are the turn signals, left for left, and right for right.
For manual buses:
Only things different are the gear lever and the black buttons.
Gear levers are the 6 gears and reverse gear (if you can't find it, push the lever vertically down and put it in 6th gear...took me a while before I realized that).
Black buttons, starting at the top going counter-clockwise: engine, parking brake off, manual neutral gear, parking brake on. Make sure that it's manual neutral and not automatic neutral.
This freed up a lot of keys on the keyboard for me. So I have the IBIS, ticket printer, changebox, wipers, doors, heater, and some other stuff on the keyboard.