Gamblin Safflower Oil. Work paint out of the brush then with warm soap and water.
Some artist use Baby Oil for cleaning their brushes. But never use baby oil as a medium, it is not designed for that use.
After initial oil brush cleaning you can do a deeper brush cleaning by using either dish soap, The Master’s Brush Cleaner, or Murphy Oil Soap.
If you have a brush that is really stiff or has hardened paint on it, us Winsor Newton Brush Cleaner and Restorer (it's non toxic and biodegradable). After soaking it in the solution for awhile, clean the brush using the methods described above.