According to this performance https://www.youtube.com/watch?v=c1zJzr-kWsI and the Wikipedia article about the song, the quote is, "One is only poor only if (you/they) choose to be". But let's stick with your version for now.
This is, after all, a Country & Western song, written and sung by Dolly Parton, so it's useful to consider the alternatives. They are, in order
A) One is only poor if one chooses to be.
B) One is only poor if he chooses to be.
C) One is only poor if she chooses to be.
D) One is only poor if they choose to be.
The first is the grammatically correct version, but it is very formal, and the repeated use of "one" is hardly ever found in common speech. Also, it doesn't scan well with the melody, since it adds another syllable at just the wrong place.
The next two are also technically correct, but each limits the gender of the subject. Of course, you can always fall back on the older use of "he" as the universal pronoun, but that has distinctly fallen out of favor. Likewise the problem of the added syllable.
The last is not, technically, correct, since "one" is singular and "they" is plural, but it is the sort of construction an informal voice might use to avoid the problems in the other three choices. And it fits the meter of the tune as well.