I'm answering the question "Have you ever been a citizen or national of any other country? Country of Citizenship / Nationality"
I used to have an Artist (O-1B) visa for the USA from 2013-2016. Does this mean I used to be a US Citizen?
No, a visa does not make you a citizen.
(Nor does it make you a national).
Henning Makholm's answer is correct, that your O-1 does not imply that you were a US citizen or national, but a stronger statement is possible: it does imply that you weren't.
Having a US visa means that you are not a US national or citizen because people with US citizenship or nationality may not receive US visas. See the US Foreign Affairs Manual at 9 FAM 301.3-3:
You may not issue a visa to an individual unless you are satisfied that the applicant is an alien. An alien is defined at INA 101(a)(3) as "any person not a citizen or national of the United States."
(There are of course cases where US citizens have been given visas because the visa officer has been unaware that the applicant was a US citizen. Often, the applicant is also unaware of this. For example, a person born outside the US to a US citizen parent may be unaware of the law governing the transmission of US citizenship in his or her case, and the visa officer may be unaware that the person's parent was a US citizen.)
To reiterate: not only does your O-1 visa not make you a US citizen or national, it implies the contrary.
A visa is like an authorization for a person to enter the country on justified purposes.
If you're a citizen of this country, you're automatically authorized to enter, well, because you just can no matter what.
So only non-citizens need visas.
A visa is like a conditional and temporary ID for the visa holder in that country. A citizen has an unconditional and permanent (more or less) ID.
That's the difference.