Hog hair or "China bristle" and especially Chungking Chinese bristle are the best fibers for lasting and holding their shape during heavy use of oil paints. Synthetic fibers such as nylon, and soft natural fibers such as sable and mongoose do not last as long as the coarse bristle of hog's hair. Look for handmade brushes with bristles that spring back into place after you press against them with your thumb; flats and brights should have clean-cut edges. Watch out for "rogue" bristles that stick out or fall away from the metal ferrule, which surrounds the bristles and keeps them fixed to the handle.
Brushes meant for oil paintings are made with long handles to allow the painter to paint with more freedom and at a greater distance from the canvas. Long brushes also keep the artist at a distance from the canvas closer to the distance of a prospective viewer of the finished painting. A quality brush will have a wooden handle that is sealed with lacquer. Brush handles are also made of molded plastic. A handle with a smooth, finished surface will stand up longer against paint thinners and frequent cleaning.
Though it may be tempting to purchase a set of brushes with various tips for different texture effects, experience will prove to you that technique is mastered through the use of a few common brushes in the sizes that best fit the size of your canvas. Flats and brights are used for sharp lines and clean edges. Rounds are used for smooth lines and varied line weight. Filberts are used for filling in space and blending. If you are working on a large canvas, purchase a large brush that will help you cover the canvas appropriately. Detail brushes come in smaller sizes and are usually made of sable fibers.
Art supply companies such as Grumbacher and Winsor & Newton have been making pigments and brushes for more than a century and have earned a place among the most trusted names in the world of painting. When purchasing paint brushes, keep in mind that companies that directly serve the art community, such as Grumbacher, Winsor & Newton, Utrecht and Dick Blick will have more knowledge and reason to provide quality materials to their customers than the average arts and crafts store. Spending a little more on your brushes will save you the frustration of replacing brushes that are mass produced and make your painting experience more enjoyable.