Österback is farm nr.6 in Ömossa, located at the very place you describe. I agree with Stefan's conclusion earlier about the couples identity. Alexander and Maja Lena are noted as farmer and wife having moved to Österback in 1852, rows 18-19 here on p.253:
http://digi.narc.fi/digi/view.ka?kuid=6988759They have been married at that time. Neither is born there, Alexander is noted as coming from p.258, Bergvik nr.9 in Ömossa:
http://digi.narc.fi/digi/view.ka?kuid=6988763Alexander's parents here are the same as Stefan gave above, living on the Vikbäck torp in Lappfjärd some 20 years earlier, so that must be right.
Maja Lena again is noted as born in Lappfjärd, and in the listing of Sandvik nr.7 in Uttermossa are notes that Maja Lena has got married and moved out on 22(?)/11 1852, which again would confirm her identity:
http://digi.narc.fi/digi/view.ka?kuid=7067818So you need to look for your deeper ancestral roots at other places than in Österback in Ömossa.
The book you refer to might be Bottnisk Bygd by Gunnar Nybond, which covers the family trees of the Öman and Skogman families in Ömossa. Unfortunately neither of this couple seems to be descendants of these families. I also have copies of some other of Nybond's books, but I can't see them listed in the indexes of my books under any of the Österberg, Bergvik, Vikbäck or Sandvik names.