There is described a substantially planar self-supporting sheet (preferably an thermoplastic film e.g. formed from a polyolefin such as BOPP) comprising on at least one side thereof a substantially continuous adherent coating, thereon to create a barrier to oxygen, the coating comprising nano-particles, the sheet characterized in that: (a) the nano-particles form a substantially continuous phase on the sheet; and/or (b) the nano-particles are present in an amount of at least about 20% by dry weight of the coating; and/or (c) the coating is substantially free of polymeric binder, synthetic hydrophilic resin and/or a resin with a hydrophilic/hydrophobic character. The coated sheets have a high oxygen barrier, with preferred OTR values (at 23.degree. C. and 0% RH)<10 cm.sup.3/m.sup.2/day. Preferred nano-particles are biopolymers such as starch and the coating is preferably substantially free of wax which has been found to reduce oxygen barrier effectiveness. Coatings components of small molecules or oligomers such as biomonomers (e.g. sucrose) have also been found to enhance oxygen barrier properties of a particulate coat whether the particles are micrometer or nanometer sized. Coating compositions, coated films, coating processes and packaged articles are also described.