O-Ring Gland Design Guide

O-Rings cross section is squeezed radially in the groove between the inside (ID) and outside (OD). Static Radial Applications Common Fixed Seal
 
Axial
O-Ring cross section is squeezed axially in the groove similar to a flat gasket
Static Axial Applications Face Seals
 

Reciprocating
Reciprocating seals refer to seals used in applications that slide back and forth. This motion introduces friction, which creates design considerations different from those of static seals.

O-Ring may be housed in a groove (rod seal) in the cylinder wall instead of a groove in the piston surface (piston seal) without any change in design limitations or seal performance.

Dynamic Reciprocating Applications, Moving Piston/Cylinder