They are not the same here though.
"Even if it is raining" = we will go whether or not it is raining. ("Even if" = the thing might happen.)
"Even though it is raining" = it is definitely raining and we are going anyway. ("Even though" = the thing is happening or will definitely happen.)
There are many circumstances in Norwegian, where 'om' is used like 'about', 'on', 'if', 'in' and other small prepositions in English, often with other words (e.g. selv om, om kvelden, om bord). These are generally things that you just need to learn.
'Hvis' is used for 'if', usually at the beginning of a prepositional phrase, and without directly connected words. In this context, 'hvis' and 'om' are generally interchangeable.
I think that one could sort of use a rule of thumb that if it is checking a yes/no condition, use 'hvis', and if you could substitute 'whether' or 'about' and get a similar sentence, use 'om'. Other than that it may be regional or subject to use exceptions, or interchangeable; it's probably safer to use 'om' than 'hvis' if you are not sure. p.s. I've been living in Norway for 5 years, and I'm still not always sure.
That depends upon English usage... "Even though it is raining, we go", means 'it is raining, and we are going, anyway'. "Even if it is raining, we go" can mean either, 'it might rain, but we are going, anyway' OR, 'it is raining, and we are going, anyway'. Norwegian doesn't really distinguish between them. If you do need to distinguish, you need to phrase it differently.