If you received Pay Stubs from your employer and the work was legally auhorized, you need to say yes. Also say yes, if you got a W-2 from the employer. Unauthorized employment is not allowed in F-1 status, thus inadmissable. However, in a long run, it may effect your permanent residency and citizenship where you have to disclose such factors, if any. Since you did have an OPT but never used, the answer should be NO. 'worked as a US citizen....?'---unclear.