Has anyone been to Oslo? I'm going on a last-minute work trip tomorrow. I'm staying near the Nationaltheatret Station which I understand is a direct train from the airport?

Is the train easy to navigate? Our work travel people got a quote of £216 for a one-way airport transfer

(although I'm sure a cab should only be £70-odd)
Anyone know any decent restaurants or things to see in that area? It looks easy to walk to the waterfront.
I understand it is cold..
Just back from Oslo last week... how long do you have there?
There are two main train options from airport to Oslo central station - the express train at £14, or a standard NSB train which takes a few minutes longer and is a few pounds cheaper. If work are paying, get the express. You'll then need to probably change to another train or tram to get to where you are, or walk - it's only about 15 minutes from the central station in a straight line(ish). Pretty much everything on trains etc. had signs with the English translation, same on the train - the automated announcements would play in English after too - so you hopefully shouldn't find it too hard to get around.
I found that most things in Oslo are quite expensive, and they keep "work hours" - so most "tourist things" aren't open after 5/6pm. The opera house roof is however a good suggestion - some nice views of Oslo, although I did it during the day, so can't comment on what it will look like at night.
Where you are staying, directly south west ish towards the waterfront, is a brand new quarter of Oslo - I'd broadly compare it to "Canary Wharf", in that there are lots of offices and expensive apartments, and restaurants to serve the busy office workers! Don't expect much change from £25 for a main course to eat somewhere nice.
If you go East from where you are, heading back towards central station, is the main centre - so lots of places to eat and shop.
If you have time, there are tour buses (
http://www.visitoslo.com/en/product/?TLp=181656) organised from outside the city hall - you can pick different length tours depending on what time you have, they start at 10:30am (ish) and the shortest is about 2 hours. It takes you to the big ski jump slope (
https://en.wikipedia.org/wiki/Holmenkollbakken) which is pretty amazing to see, and then to the Vigeland park on the way back with lots of statues (
http://www.vigeland.museum.no/en/vigeland-park) - both are worth a visit. One free attraction I quite enjoyed was the city hall, which is quite near you - doesn't take long to go round and see it either.
Last thing I'd add, is that most tourist places seem to close on a Monday, which was a bit annoying.