My two pennies worth of thoughts on doors, based on some experience from making them.
Indoors or outdoors, doors (couldn’t resist) are prone to warp. I imagine it’s because of differences in temperature and humidity between their lower and upper parts. For an external door this means that it is important to provide some form of coverage against rain, above the door. If that can be sufficiently extended to also protect the lower parts of it against downpours, then so much better; though, of course, that isn’t the general case.
Oak is very resistant against the effects of moisture, and will – similar to other wood species – as long as it can dry up between becoming wet, last for a very long time, though perhaps not aesthetically pleasing (see attached photo of an oak pier).
As sapwood is more susceptible to rot, it can be a good idea to have it at the upper parts of an external door, where it is less exposed moisture.
Having left two external doors without any coating, I’ve learnt that they swell and become ugly, and that’s not good even if they last. Consequently, I nowadays use polyurethane varnish to protect their looks and function.
The attached link is to a page that discusses finishes for outdoor projects.
Best of luck