A few of my bristle brushes have splayed out so I experimented on how to reshape them.
On most bristle brushes, the hairs have a slight natural curve to them. But with use they get out of shape. In "Painter's Handbook: Revised and Expanded" by Mark David Gottsegen, he suggests wrapping the bristles in a wax paper sleeve (after cleaning the brushes of course). I couldn't get the paper to stay put so I taped the wrapped brushes to a piece of old mat board. Then I smushed the brushes with another piece of mat board and clipped the edges.
This did reshape some of them, a few didn't seem affected, and one is just too far gone. It was nicely reshaped the next day, but as soon as I got a bit of paint and OMS on it the bristles splayed out again (too much dried paint up in the ferrule probably from a rushed day). If I try this again, maybe I'll tape the wax paper sleeve to the ferrules so the brushes doesn't slip out.
Next I'll probably try what The Master's Brush Cleaner company recommends with their product (leaving a clean lather in the bristles to reshape it, let it dry, and then shake the powder off when you're ready to paint). This would be fore bristle brushes. I have no idea how or if this would work with sable brushes. (This isn't the only cleaner out there, do some research and see what you like).
Side note: If you do an internet search you can easily find many ideas for how to care for natural hair oil painting brushes. They all involve these general steps: wiping excess paint off the brush onto a rag and getting the paint out of the bristles using some sort of solvent (getting pretty much all the paint out of the brush before you lather it up so you're not sending paint down the drain or washing your brush with paint and soap), then cleaning and reshaping the brush with some sort of soap/brush cleaner.