ROLLAG/HALVERSON in Rollag, Norway Emigrated to US

I’m looking for information on my family. Ole Halverson Rollag (born 1837) in I think Telemark, Norway. I’m not sure if the name Rollag was actually his or if it was Halverson and the family changed the name to Rollag when emigrating to the states (we think to South Dakota). I was told that our lineage comes from a small village not far from Rollag. According to Ancestry, Ole’s dad was named Halvor Torgeirson (born 1797). I’m looking for information on where we come from, what village, and more info on when we went to the states. Thank you!