There is no "normal" about Swedish surnames until 1901. Until then your surname was what you said it was, at any given moment. At the yearly household examination, or at your wedding or your child's baptism, you just told the vicar that from now on you wanted to be known as ... and that's what he wrote in the record.
The 1901 Name Ordinance (it wasn't an Act) didn't differentiate between patronymics and other kinds of surnames. It just stated how a family name was acquired; unique names were basically patented while non-unique names (like patronymics but also town names and soldier names) could still be used by anyone - but you couldn't change around as freely as before.
In 1921 women were obliged to change to the husband's name on marrying. But it was only in 1963 that it was made mandatory to have a family name (before that you could use patronymics that still changed for every generation - my maternal grandfather's family still does it).
More about Swedish naming customs:http://web.comhem.se/~u31263678/genealogy/Names.pdf
In this case the man was in the Navy ("båtsman") and was thus, like any military man (bar officers naturally) given a new surname.
"Öhman" would of course refer to the part of the parish he lived in; many military names were construed like that. But you shouldn't read too much "meaning" into Swedish names - a name like e.g. "Bergqvist" meaning "mountain twig" is quite common... The first part of these so-called "town names" (yes, Öhman is a town name although it was used by a military man) quite often (not always!) had "meaning" in the sense that it referred to something tangible, like e.g. the village you came from, while the second part of the name followed a traditional pattern and was chosen more because it "sounded" nice than that it gave meaning to the name (mountain twig?!).
In any case, it wasn't unheard of that a son assumed a different surname than the one his father had. I've seen a case where four brothers all changed from their father's name - into four different names. So when it comes to surnames in Sweden until 1901, most things could happen - and there was no "normal".
However, first names followed norms and while it is possible, I think it quite unlikely that Nils Erik's father was Erik OlAf; OlAf is the Norwegian form while the Swedish form is OlOf.