You will need the following to clean an oil brush.
A brush soap, or cleaner, whatever you call it. It could be any brand, as long as its effective. The current brush soap is from Curry's, the art store.
These kinds of detergent soaps are also fine, actually. They work just as well.
Start by using the brush and brushing it on the soap as you would normally do when painting. Make sure to get paint on the soap, you can always wash it off later.
Squeeze the brush between your fingers outwardly, so you're taking the paint out, which had become easier to come off from the soap.
Do the same thing again.
Repeat squeezing motion. After repeating several times...
Your brush should look clean and decent, and slightly wet.
Keep your oil brushes in a bottle of oil, in a moderate environment. The oil can be any type of oil (even cooking oil!)
Change the oil regularly, maybe once every two to three weeks. When taken out, the brush is ready to use.