I use three basic categories of brushes for oil painting: bristles, white nylon flats, and sables. The bristle brushes are made from Chinese hogs. These inexpensive brushes are stiffer than the others, and are useful for blocking in big areas, and manipulating thicker paint.
Bristle brushes are good for keeping your painting direct and simple, with a greater likelihood of getting soft edges. Generally the stiffer the brush the softer the edges. I like flats and filberts. Brights are like flats, but shorter. The Silver company makes an "extra-long filbert," which has a wonderful touch.
White nylon flats are excellent for detailed painting of architecture and technology. They’re available in widths as narrow as 1/4 of an inch to 1 inch wide brushes for laying in a transparent wash of thin paint. A flat brush should have a chisel tip, which you can use for a wide stroke or a thin line.
Nylon brushes are fairly inexpensive, but they don’t last long. Most of the manufacturers, like Dick Blick, Grumbacher, and Simmons make versions of these, and they’re all pretty good.
For small detail work, I use Kolinsky sable rounds. They're made from the reddish tail hairs of a kind of weasel, not a sable (which is a kind of marten). They’re intended for watercolor, but they work equally well for oil. These are the most expensive brushes, but they respond very sensitively to detail work.
The Winsor and Newton Series 7 has always been the standard, but they’re overpriced, and you can get good Kolinsky sables from Escoda, Silver, and other makers. Most often I use Raphael 8404, which is fairly priced, and as good as any others.