It’s a doughnut-shaped loop designed to prevent the passage of liquids or gases. It’s one of the simplest precision mechanical pieces ever produced, and continue to be one of the most widely-utilized sealing products. An O – Ring is a circular band with circular cross section to arrest leakages between two faces or over shaft and hole. They are designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface.