Yeah... While Walnut allegedly works well outdoors... the beauty fades fast....
I think Locust, Cedar, Cypress, and others would be great choices too....
Oaks also can do well outdoors... They don't rot much... but they do move a whole lot with the moisture changes... Finishes tend to flake off pretty bad... They are also quite porous - so you gotta watch out for stuff getting into the wood...
If I was going to use Oak outdoors - I think I would try limit them to places where they would stay out of the weather....
Thanks