Are these rake brushes suitable for oil painting
You can use any brush for any medium as long as you achieve the effect you are looking for. The only suggestion I would have is that if you have used a brush for oils you can not then reuse it for any other medium. It must stay an oil brush as it will be contaminated with oil ad this may affect the paint or surface for watercolours.
The rakes are made from a synthetic variegated hair. Which are great for grasses and fur in watercolour. They may be a little too soft for the heavier oil paint, but it depends on how thick it is applied.