Not a chance I'd leave it unprotected. When oak gets wet, you'll get those black stains around any joins where water lingers, and especially where grain is exposed. (I'm assuming there's no end-grain on show though.) Those black stains are nigh on impossible to get rid of once they've happened.
Tbh, OP, my expertise is in internal finishes as I don't really get involved with external doors or their finishes. However, if you want a nude look, there's a company called Broadleaf (broadleaftimber.com) who sell a Nude Oil. I've used this on floors and it hardly colours the timber at all. I use it maybe two or three times a year and I'm consistently amazed at how it doesn't colour the timber. It's an emulsion which you can apply with a brush, or short pile mini-roller. It's only sold as an internal finish on their website though, so I assume it probably wouldn't cut it as an external until the door is further protected from the elements.
Any oils will deepen the shade of the timber. I've not come across one that doesn't yet, no matter how much they promise. They do soak into the timber though and give a hydrophobic surface payer - but they're not apply and forget. You will need to renew them perhaps yearly.
You could use a two-part water based lacquer from Bona e.g. Traffic (bombproof floor protection for commercial environments - but I often use it for domestic, as it leaves a nice pale finish (though a few shades darker than bare timber.) With all water based finishes, the door should be sanded down to say 120 or 150 grit to close up the grain. Then apply one or two coats of sealer/primer, knock back raised grain with 180/240 grit, then two or three coats of two part finish. A bit time consuming, but it would leave your door well-protected.
Sorry, but the non-colouring external oak finish is yet to be invented (or it has and I haven't heard of it).