From Oslo Central Station approx. 3 hours. Trains often terminate at Larvik, and the last leg to Porsgrunn is then serviced by bus.
Both Nor-way Bussexpress[url=http://www.http://web2000.reiseinfo.no/Nor-way/Nor-way/Grenlandsekspressen/eng/default_eng.aspx]]and the slightly cheaper, but less frequent Konkurrenten[url=http://www.konkurrenten.no/[/url]]operates buses between [[Oslo[/url]] and Porsgrunn.
[wiki=851fb3b31d1c330cab49d90f3e9c9784]Skien[/wiki] Geiteryggen Airport is serviced by daily flights to [wiki=d36096a10b82aadd44de1efb13058c0b]Bergen[/wiki], [wiki=dd755cfcbe227d7271b91c1be176fe6f]Stavanger[/wiki] and [wiki=09346c14b310f8f03065d78fbfff351e]Molde[/wiki], as well as three times per week to [wiki=fcfff492e00727b63cf5dff9f59bc2a4]Stockholm[/wiki], Sweden. For more information, visit Widerøe [http://www.wideroe.no]. There is frequent busses between Skien and Porsgrunn, and the trip with a Taxi should not be too expensive either (relative to norwegian prices).
Sandefjord/Torp (serviced by Ryan Air, KLM, Widerø and others) is also a good option to travel to Porsgrunn. There is a bus (Telemarkekspressen) and train connection and it takes about 90 minutes to get to the city center.