The O-ring is the most common way of sealing because it is not space-intensive and its assembly is very simple. With the proper construction of grooves and suitably chosen material, it performs its long-term function both in static and dynamic use within the temperature range allowed for the respective rubber material.

The O-ring is a closed ring with a circular cross-section, made mainly of elastic rubber material, the so-called elastomer. The size of the O-ring is defined by its internal diameter (Ød1) and cross-section (Ød2).

O-rings are most commonly made from different types of rubber by vulcanization of the blank in die molds or seamless injection molds.

The sealing effect of the O-ring embedded in a suitable groove is achieved during assembly by compressing the shape of the circular cross-section of the ring (d2) elastically to the elliptical profile. The compressed ring seals the gap between the contact elements or the surfaces and the bottom of the groove.

The sealing effect is thus the result of changing the shape of the O-ring profile from the circular to the oval. The degree of this change depends on the depth of the groove (t). The compressive force thus created is also referred to as overvoltage, which ensures the system's leakage. The cross-sectional diameter (line cross section) d2 must always be greater than the depth of the groove (t).