I just replaced the seat and back of an old cast iron garden bench with some reclaimed solid oak wood. Whoops. The cast iron is reacting with the tannin in the oak and giving black stain, but it was such a p.i.t.a. job it can bloody stay that way
Also there is a difference between American Oak and European Oak. The latter is more weather resistant ISTR

Make sure you use brass fixings, things like steel screws will simply rust away. If using screws always drill correct pilot and clearance holes and drive a steel screw in first before replacing with a brass one. Use a traditional wood screw to form the threads for the brass screw rather than the more modern twin threads and self drilling screws.

Richard