kristiansand is rather a long way from the hardanger area (250km approx), which is maybe what you mean by "the fjords"? it can be done but you would probably need to do a 2 day tour with a stay overnight. But a day tour is possible to Flekkefjord which is 120km away, & although not in such grand scenery is still very attractive. Kristiansand is itself on a fjord, and you can take a ferry to the islands, or hire a boat. Have a look at
http://www.krsguide.com/?node_id=5&menu_id=5
oslo is about 4hr away by train
http://www.nsb.no/internet/en/ & a little more by bus
http://www.nbe.no/ .
There are flights to Oslo-Gardermoen (with SAS-Braathens), but expect to pay around �100-200 for a day return.