If you have an accident, it can happen that something of the bus breaks, e.g. a light or an indicator if you hit a street lamp, or the wipers, doors or heating and in serious accidents even the electronics or the engine. These defects are implemented in the game (e.g. the doors do not open anymore (or very slow) when they are broken). What exactly breaks depends on the strength of the accident. The repair function in OMSI does not help to repair the damages from the Bus company simulator. The buses must be repaired in the garage of the company.
Of course there are still the normal defects of OMSI (e.g. a missing mirror at the 3 Generations bus). These damages can still be repaired with the OMSI repair function without costs.