Yes, you use it to talk about location, it just means you say where something is!.. also:
Use om to mean "about", but never with approximations, with that I mean, you cannot use om to refer to the number of people in one place, for example. .(There are about/around three book in my backpack- don't use om), so:
• A book about Einstein: en bok om Einstein. • We talk about you: vi pratar om du
When talking about time, you use om to mean "in"/ "within":
• I'll be there in four hours: Jag kommer att vara där om fyra timmar, and yes!, you can also use i here! •In two decades: om två årtionden.
Ufff, one more! om also means "around", referring to a place, and never approximations of quantities, as explained above, Here runt and kringare also possible:
• Marie has flowers around her house: Marie har blommor om/runt/kring hennes hus.
•I put a fence around the yard: Jag satte ett staket runt gården
It means "a" when referring to a daily or yearly frequency, here the "formula" is om + noun(en- et), so, let's see: •I go to school twice a day: jag går till skolan två gånger om dagen.
•Jag reser till England fem gånger om året. You can say "per år" when you don't talk about a daily or yearly frequency you say like that: "i + noun(en-et)" or "per + noun":
•I work three times a week: Jag arbetar tre gånger i veckan/ per vecka
• My father likes to walk through the park four times a week: Min pappa gillar att gå igenom parken fyra gånger i veckan/ per vecka
It means "of" when you mean possession, but it is not much used!
I am curious if those flowers are 'around the house' on the inside, like 'she has flowers around the house, decoratively arranged in vases' or outside, making the circumference of the house like 'she has planted flowers in beds under the windows all around the house.' Can it mean both/either?
So if I get that right, in the Sabaton's song "Carolus Rex", when he sings "Över Norden jag härskar" (so much for the V2 rule, but it's a song so i guess it doesn't matter x) ahah) he's using the definite of the old variation, is that correct? (probably because Charles XII was from the XVIIIth century and using it himself, come to think of it xD)
As you guessed, the sentence structure is just poetic license. It's common in lyrics and poetry to ignore some grammatical rules for poetic effect, just like in English.
Norden is a proper noun - it's our name for the Nordic countries, i.e. Sweden, Denmark, Norway, Finland, and Iceland. So it's derived from the definite of the old form, as you say, but I must note that it shouldn't be confused with the noun norr/nord. :)