Glossiness in oil paintings is achieved through the painting medium used. The mixture of varnish(es) with the linseed oil and turps determines how the paint will look when dry. There are a number of premixed mediums you can buy, and you can make your own from recipes in artist reference books. Linseed oil on its own does not have the gloss or weight to settle into a smooth surface, that is the job of the varnish (or other kind or resin.)
Another major factor in eliminating brush strokes is the choice of brush you use when painting. White bristles are firmer and will leave brush marks, the softer (red) sable brushes and fans will be smooth. Painting with a soft brush and then lightly flicking over that with a fan brush to blend is a classic technique.
It is a mixture of medium and brush technique that gives you that super smooth finish.
edit: to downvoter, could you explain why?