I was going to ask you if you knew where they lived when they died, when I found the replies Judy Baouab gave you in 2009. Did you ever order the inventories of Oskar's and Nelly's estates, bouppteckningarna? It is still your best bet to find the names and adresses of their children and other heirs.
Anyway, as both Oskar and Nelly died in Gualöv I made a quick search of the Swedish phonebooks online and found one man called Lang living there. As it is a small community of 500 inhabitants, chances are he is related to them. To find him go to www.hitta.se
, in the first box "Vad?" enter the name Lang, and in the second box "Var?" enter the location Gualöv. You will get both his phonenumber, his adress and a map of where he lives in Gualöv.
Best of luck