Not the best choice for longevity outside regardless of treatment---unless it is white oak. All the stock railings I have seen at stores have been red oak.

Painting any kind of oak will look like painted oak---which is the single worst look of which I know.

Using a white oak railing---if you can find one premade----and coating it in three coats of marine spar varnish is the best way to get a couple of years before recoating will be needed. You can stain(or not) the wood, then use a spar varnish woth UV protectants added. That will give the best long term protection---long term dependent on how much direct sun the wood gets.