The invention relates to a coated O-ring for forming a seal, in which the core of the O-ring comprises a polymer and/or synthetic rubber, and the coating comprises an inorganic coating with a thickness between 10 nm and 1 μm, resulting in an O-ring that shows a lower permeation than an uncoated O-ring and where the coating shows little or no cracking when the local curvature of the O-ring in any direction is changed by 20%.The coating is preferably applied by Atomic Layer Deposition, but also other deposition techniques such as (PE)CVD, E-beam evaporation, and reactive evaporation may be used.A further elastomer layer may cover the one or more coating layers for further mechanical and/or chemical protection.