I've just moved from an older US MacBook Pro running Mavericks, and I've been able to write the Norwegian 'æøå' characters by using alt-', alt-o and alt-a.
On my new US MBP running Yosemite, all of these work, except for alt-a, which just writes '¯' instead of 'å'. I know of the option to hold down keys to get a list of alternative characters, but the alt-variation is much faster to work with, and I'd really like to get it to work. I've installed the latest OS X update.