After arrival at the airport
Oslo's main airport is located at Gardermoen, around 50 km north of Oslo. On your right hand-side after coming out from the custom area you will find the train station. From there it takes about 25 min by train to the city center, e.g. to the Nationaltheater station which is usually the most convenient for visitors of UiO. The Airport Express train Flytoget goes every 10 minutes and costs 190 NOK per trip. Alternatively you can buy a ticket with a regular local NSB train (either R10 to Drammen, R11 to Skien, or L12 to Kongsberg) at the cost of 93 NOK per trip (buy a ticket for four zones). In any case, be aware that you must buy a ticket before getting on the train. There are also busses, but we recommend taking the train. Taking a taxi should definitely be avoided, as the price for one trip can only be considered as outrageous.
Getting to the University's main campus and NH Abel's House
Take the metro. Metro stations in Oslo are indicated by a white sign with a blue encircled T ("T-bane" in Norwegian). Here is a map of all metro lines. All lines pass through the stations located in the city center (Nationaltheatret, Stortinget and Oslo S). To get to Blindern from one of these choose west bound line 4 Vestli via Storo, or line 5 Ringen via Storo, or line 5 Sognsvann to Blindern station. The metro trip takes 7-8 minutes. The campus is situated on the little hill top strait forth as you get out of the metro. The red brick buildings should be easy to spot.
Take the tram. Here is a map of the tram lines. Both line 17 Rikshospitalet and line 18 Rikshospitalet go to the University campus. The stop, which is called Universitetet Blindern, is on the upper side of the campus. The tram trip takes around 10 minutes. The stop closest to Thon Hotel Europa is Holbergs plass; the one closest to Scandic Karl Johan is Tinghuset.
Walk. With a normal walking speed it will take you around 40-45 minutes to go to Blindern (say from the National theatre).