I will be travelling to France in April and playing my oboe in outdoor ceremonies where there is a chance the temperatures could get as low as 5 degrees celcius (we are playing in a dawn service in northern France). Having only played my oboe mostly indoors in Australia I'm a bit worried about how my oboe and reeds are going to react to the possible cold weather. Any advice or tips?

Even if my oboe copes ok, I'm not sure my poor cold fingers will!

With luck it will be warmer than that - I live in southern England, not much over 100 miles from N France,& have been there even earlier in the year & it has been warmer than you are expecting but not guaranteed.
The important thing is to try & avoid sudden extreme changes in temperature for the instrument & swab it out frequently as in cooler temperatures there will be more condensation forming in it. Also warm it up gently & try & keep it warm, maybe under clothing,when not playing.

your oboe is wood I take it; the plastic ones are great for this. (cold weather without fear of cracking)
even with careful warming up I would hate to play in those conditions

Now, I've never done this (it freaks me out too much), but I have seen every professional oboist I know do it. To warm up their oboe, they but the bell and the bottom joint together and put it under their armpits, and put the top joint under the other arm. Hope that can help.

If I were you I would not play a wooden oboe outside. I'm am really afraid of mine cracking so if I must play outside I use my plastic student Yamaha. Music stores usually rent student yamahas for pretty cheap and they are good oboes. If I need to go on a plane with an oboe, I usually rent a student yamaha too, I'm just that protective of my Loree. =]

I always try to wear a jacket, even for indoor concerts where it might be cold. When I'm not playing body heat works well keeping it at a reasonable steady temperature. Make sure to bring a variety of reeds. I've noticed that when I play at different temperatures reeds react more to the difference than my oboe does. Make sure you don't warm it too quickly either.