Out-of-Service (OOS)
A condition serious enough that an inspector prohibits the vehicle or driver from operating until it is corrected. Also the rate at which that happens, used as a safety measure.
An out-of-service order stops the vehicle or the driver on the spot. It is not a citation to be dealt with later; the unit does not move until the defect is fixed.
The out-of-service rate is one of the few externally verified fleet safety measures, since it comes from inspections conducted by someone other than the fleet. That makes it harder to flatter than internally reported numbers and correspondingly more useful, which is why it appears in benchmarking.
The relationship with preventive maintenance is direct and worth stating: the defects that most commonly trigger an order are brakes, lights and tires, which are exactly what a functioning PM program and a working driver inspection process catch first.