What is the rate of taxes for someone who has worked on OPT?

On F1 and OPT you are exempt from residency for 5-years. You must file non-resident tax return. Your income is not subject to FICA taxes.
You will get exemption of $3500, itemized deduction and deduction as per tax treaty between the U.S. and your country. Read: Your U.S. Tax Return: U.S. Tax Filing Requirements for Non-Residents


Only if you tell you total income, your country and your deductions, someone can tell you about your income tax.

Tax rates start at 10%, then 15%, then 25%, then 28%, then 33%, and topping out at 35%.

You probably will not exceed 25% as your top tax rate.