Standard hair: Hog bristle (also called "chungking bristle" or "china bristle") — naturally split ("flagged") tips that hold and release thick paint evenly
Standard shape: Filbert is the closest thing oil painting has to a default — its soft edge suits the blended, glazed, built-up surfaces oil is known for; Flats are equally common for bold, visible strokes (Impressionist-style direct painting)
Softer alternative: Badger hair or badger/synthetic blends — used for the final softening pass over a blended area, not for moving paint
Modern synthetic option: Stiff synthetic filaments (e.g. Da Vinci Impasto range) now approximate hog bristle stiffness for artists avoiding animal hair, though purists still consider true bristle unmatched for heavy impasto
Handle length: Oil brushes traditionally have longer handles than watercolour brushes, made for painting upright at an easel, at arm's length from the canvas