If you learned old Norse you would probably not understand shit of modern norwegian, bar a few words. Same with other way around.
The danish people buttraped our language to fuck.. Its pretty vile if you ask me, i wouldnt mind learning old norse and getting a law much like the icelandic one.. You know, the one where you cant loan any damned words from an other language.
I don't know much about Old Norse though, except it lacked a lot of syllables we use today, and it also had some we dont.. The few words the old norse and norwegian share, were almost certianly pronounced very differently because of this. Still some survive in the original form through dialects, like my own, but they arent written words.