First thing:

1- What nationality is Olivia? I say Australian, at heart, British by birth, and American for practical purpose.
So what is her nationality officially?

Technically, any country which issues you a passport, grants you nationality at the same time.

Therefore, she can have 2 or 3 nationalities (British, US and Australian).

Feel free to add to and/or correct this! and to add another element of trivia!

Technically she is Brittish but she picked up the Austrailian accent.

"Olivia never considered herself a country singer," said one music critic "and that hurt her later on when she switched from country to pop then rock and back again to country-pop," he continued.
He said "if Olivia would have considered herself a country singer when she first started having hit records in the US, then she would have had a country career longer."

AGREE or DISAGREE?

She is an Australian but spends a lot of her time in a country that has been good to her.