Yes, as was mentioned above red oak is a poor choice as it is not weather resistant at all.
If you are using an "oak" then use white oak which is harder and much more resistant to weather and rot.
Two other great domestic choices which are even more weather and rot resistant and maybe even harder than white oak are black locust and osage orange (hedge apple). Both are a bit yellow when freshly planed but will age to a brown shade with exposure to some sun.
There are also many South American imports which are better than any of the above woods and very hard. I've used ipe many times and is is ridiculous. Heavy, hard and durable. Great stuff. You can get ipe and many others at http://www.advantagelumber.com/
and I'm sure ipe probably available through some of the better home center places.