Looking for lost relatives.

I am looking for relatives from Norway. My Great-Grandfather moved to Chicago from Norway in the early 1900's. I believe his last name at the time was Malmedal (Not sure about spelling?). After moving to the USA he changed his last name to Johansen. His first name was Oscar & he had a son named Barney. If anyone has any information where I can go to try to get information please let me know.