(impersonal pronoun, indefinite) One thing (among a group of others); one member of a group.
Any one of the boys.β The big one looks good.β I want the green one.β A good driver is one who drives carefully.
2
(impersonal pronoun, sometimes with "the") The first mentioned of two things or people, as opposed to the other.
She offered him an apple and an orange; he took one and left the other.
3
(indefinite personal pronoun) Any person (applying to people in general).
Oneβs guilt may trouble one, but it is best not to let oneself be troubled by things which cannot be changed.β One shouldnβt be too quick to judge.