Offset is the term used for two different measurements related to the running gear of a vehicle.

In a steering system, offset is the distance between the centre of the tyre contact patch and the point where the steering axis hits the ground. This is also known as steering offset or scrub radius.

In wheel design, offset is the distance between the centre plane of the wheel and the mounting face which sits flush against the wheel hub. If the mounting face is inboard of the wheel's centre plane, the wheel has positive offset (also known as outset). If the mounting face is outboard of the centre of the wheel, there is negative offset (also known as inset).


In Formula 1 racing, the option tyre is the softer of the two tyre compounds available to the teams during a race weekend.

Because option tyres have a softer, grippier compound they generally produce faster lap times but are not as durable as prime tyres. However, the effect varies with the track layout and temperature.

During a dry-weather race, drivers are required to use both prime and option tyres. The rule does not apply in wet-weather races, when wet tyres are used.

See also prime tyre

Overboost is a system fitted to some turbocharged cars which allows extra boost pressure to be developed for a short period to enhance overtaking ability.

High levels of boost can lead to unreliability, but overboost allows higher boost in short doses. This improves performance where it is needed most, without making the engine unreliable.

O Oxygen

O2 Oxygen molecule

OBD On-board diagnostics

OEM Original equipment manufacturer

OHC Overhead camshaft

OHV Overhead valve


An oil cooler reduces the temperature of lubricating oil. Oil which gets too hot can break down and lose its lubricating properties, and an oil cooler can prevent this.

Oil coolers are most commonly used to cool engine oil, but similar systems can be used to cool transmission or axle oils, or power steering fluid.

At its simplest, an oil cooler can be nothing more than a length of tubing which is placed so that it is cooled by passing air. For greater cooling effect the system can incorporate an oil-air heat exchanger which looks similar to a car radiator and works the same way.