An O-ring achieves its sealing effect by being pressed into a groove (groove = installation space into which the O-ring is inserted).
The groove is always designed so that the cord thickness of the O-ring is greater than the depth of the groove.
As an example, the O-ring of a standard flange seal is designed so that it has a cord thickness of 3 mm and the groove depth is only 2.3 mm. The O-ring is therefore 0.7 mm larger and this is the part of the O-ring with which the sealing effect is achieved by pressing.
The elastic O-ring is therefore pressed from its original round shape into an oval shape. Compressing the O-rings builds up pressure, which ensures that the sealing gap is securely sealed for the long term.