Larvik is a city in Vestfold.
The Norwegian state railways (NSB) has departures for Oslo once every hour throughout the day, journey time for Oslo is 2 hours 6 minutes on most departures. There is also two daily trains to Stavanger, the western terminus of the southern railway line.
You should definately visit The Culture House Bølgen (means the wave in English). 
For information about everything in and about Larvik email: [email protected] sms: +4794261971
Twitter: twitter.com/visitlarvik