It represents the name of the United States in colloquial language (United States o' America), as the (o') with apostrophe is inflected, representing "of".
In formal writing, United States of America is used
United States o' America, there is a margarine called Land o'Lake (which means Lamd of Lake).
United States o' America is the colloquial form for United States of America, in which the "o'" with an apostrophe represents "of" in the inflected and short form.
United States o' America is a great popular country!