"some family information" doesn't really tell me much about your source. I'm trying to find out how much you understand about tracking your Norwegian ancestry yourself... and therefore to know how much I have to explain.
Yes, he could have been called Amund Bru -- Bru being a farm in that area where he had lived or had family history. But his name was actually Ommund Mikkelsen -- and that spelling is not definitive -- stay flexible, it might appear differently in every record you find for him. The vital thing is that is sounds the same and represents the same man... much like the junk mail you probably also receive with your name butchered. It has no effect on who you are.
The 1865 Norwegian census shows him living in Odda as a widower with his youngest son still with him -http://digitalarkivet.uib.no/cgi-win/webcens.exe?slag=visbas...
Published histories of Ullensvang/Odda indicate that his children all appear to have emigrated in the early 1870s to the US -- that's where the Garretson was noted.
Was that in Minnehaha Co.? There appear some likely Edmensons and Amundsens in the 1880 US census... could be his children and their families.
The old man himself emigrated from the city/port of Bergen in 1887 - the database lists him as at least 10 years younger than he actually was. http://digitalarkivet.uib.no/cgi-win/webcens.exe?slag=visbas...
There are many resources online and many more available on LDS microfilms that you can use. I can point you to them if you are ready to research. I also had ancestors from that area and have previously used many of them.