A method of manufacturing a magnetic recording medium comprises sequential steps of: (a) providing an apparatus for manufacturing the medium; (b) supplying the apparatus with a substrate for the medium; (c) forming a magnetic recording layer on the substrate in a first portion of the apparatus; (d) treating the magnetic recording layer with oxygen gas in a second portion of the apparatus at a sub-atmospheric pressure and for an interval sufficient to provide the resultant medium with at least one of the following, relative to a similar medium manufactured by a similar method but wherein the oxygen treatment of step (d) is not performed: (i) a more negative nucleation field (H.sub.n); (ii) increased remanent squareness (S.sub.r); (iii) increased signal-to-medium noise ratio (SMNR); (iv) narrower switching field distribution (SFD); and (v) decreased thermal decay rate; and (e) forming a protective overcoat layer on the oxygen-treated magnetic recording layer in a third portion of the apparatus.