S/S North Star to London, then?

I need some guidance. I am trying to Learn about my great great grandpa, John Amb. He was born Johannes Mikkelsen in Norway in 1849. He left Oslo on the S/S North Star, with a final destination of Cedar Falls, Iowa, on April 22, 1869. He arrived in London on May 6, 1869. I am assuming from there he hopped on another ship. I am trying to find the list from the ship he would have went on. My assumptions from doing some reading are that a) The ship from London and the S/S North Star were part of the same company. b) He took a Great Lakes arrival and was probably off the ship in either milwaukee or chicago. If you have any information or guidance on where I should look next, that would be great. In the meanwhile, I will keep reading! Thank you,