I heard that oak won't bonsai properly, never tried it though....
Ash and beech work great though, I've got one of each that are 15+years old now.
The ash was found in a broken pot behind the dustbin and the beech was pulled up as a 1 year od seedling just before the leaves came out. They get a root trimming ocasionally (5 years plus) weeding as needed and lots of watering :)