Looking for birthplace of Oluf Marinus Johnson

MY grandfather is said to have been born in Tromso (or at sea under an American flag) ,to have served to the trade of tinsmith in Tromso for 4 years and then to have been at sea for 14 years. All I know for sure is that he was in Southern Mississippi USA by 1905, working for the railroad. I have been told he had two beautiful sisters who lived in the Chicago, Ill. USA area in the early 1900's. He lists his birthday as July 4, 1873 on his railroad retirement papers and he died in 1944 in Mississippi. His railroad retirement papers lists his father as John and his mother as Mary. He gives his fathers birthplace as Norway and his Mothers birthplace as Ireland. His 3 known children's names were: Harald Sigard Johnson Olaf Marinus Johnson, Jr. Cecile Johnson That's all I know, would appreciate any help. Thanks!