Plain soap and water works extremely well after the brush has been wiped free of excess paint.
If the brush is going to be used immediately with other oil-based paints, this obviously is not going to work.
Maintain a small glass jar with well-fitting lid. Fill the jar with turpentine solvent and place a small metal coil in the bottom of the jar to rub the paintbrush against. Repeat rubbing and dipping in the solvent 2-3x. Make certain the jar is tightly closed after each use. After painting is complete , clean the brush with paper,m dip and scrape in the turpentine and follow up with the warm water/ detergent treatment. Allow the brush to air dry.
As for brushes with dried oil paint, try the turpentine method ; if not effective in cleaning the. brush, it is time to consider a new brush and better work habits. Don't feel bad: Van Gogh and Gauguin were known to be very sloppy with proper handling of their paints! I wonder the condition of their brushes.....