A single terminal building. It took us all of 6 minutes to get from the train to the BA lounge (via both check-in and security). Downside is that from the lounge you still have to go through one check to let you into the international area and then passport control and we seemed to board at the furthest gate. There are two trains you can get to the centre run by different companies so make sure you get the right one. The airport express is fast regular and only stops at Olso central station. The other is slower but considerably cheaper and also continues part the central station which may be handier for your hotel.