We’ve got oak doors and windows, and I’d second the above that if you’re not in to maintenance, avoid them. It’s all in the prep, and preparing oak is very labour intensive (read knackering if you do it yourself).
We’ve used sikkens and it’s pretty good. Bit of a pain though being a multi coat / 2 product system IIRC. I think we’ve got about 2-3 years out of a coat where not in direct sun or protected.
Prior to that we’d oiled them and we were never happy with the lack of longevity.
Internally on the stairs we used another sikkens product (sorry can’t remember which), but I’m personally not keen on the finish.
Unless you put some type of varnish on (not good on oak) welcome to the world of having your doors “striped” by wasps chewing the wood up for their nests. You can just hear the little buggers chewing away…