Old Icelandic vs. Old Norwegian

I've just started learning Old Norse from this site, and it says that
it teaches Old Icelandic. However, I'm curious; what are some of the
main differences between Old Icelandic and Old Norwegian? I'm
learning Norwegian right now also, and I'm wanting to compare the
present day language with that of the Vikings... so I'm just
wondering if there's any big differences between O.I. and O.N.

Thanks a lot!

~Sal