Just got an email from the employer:

" Our advertising states that: Upon application, candidates should be in the 
possession of proof of the legal right to employment in the United States. 
Verification of legal right to work will be required between the time of final
 selection and hiring and is absolutely essential to ultimately being hired.

I need to know if you are in possession of the legal right to work in the 
United States."

Should I say yes to this question? Do I need to explain my status in OPT? Does
 this question equal to "Are you legally authorized to work in the US"

yes, you need to tell them your OPT status.

Do you think I need to explain to them that I need to apply for H1 after my OPT expires at this stage?

How do I word my reply will be appropriate? I don't want to scare them away if they don't know much about H1 issue.

就说 I have a one year working permit starting xxxx. Before this working permit ends, I need sponsorship of H1B working visa from your company.

Absolutely! The prospective employer will appreciate the fact that you explain to them your situation. OPT does not constitute the "legal right" (which represents more long-term) to work in the States.

就是yes吧，然后把opt解释一下