I think you mean the Sandefjord Airport Torp
, located 110 km in the south-west of Oslo ... it is the airport of Ryanair ...
The Torp Express bus service
(www.torpekspressen.no) corresponds with Ryanair's Torp - London, Glasgow and Frankfurt Hahn flights. Buses arrive Oslo Bus terminal - Galleri Oslo (appr. 2 hours travel). The buses leave the Bus Terminal approx. 3 hours before flight departure. The busses stop at Skøyen, Lysaker, Høvik, Asker and Drammen.
The return-ticket is 28 EUR ... singel-way ticket is about 16 EUR.
You may also take the train to Oslo. From Torp airport you will have to take the bus to Sandefjord Railway station, and from there take the train to Oslo. The bus service runs on an hourly schedule from early in the morning until late in the evening. Reduced service at weekends. The name of the bus company is Telemarksekspressen
It is also an option to take a taxi
between the Torp Airport and Sandefjord Railway Station. The journey by taxi takes approx. 10 minutes.