I am employed under OPT status under F1 visa and my annual income is $41,500. What would be the tax structure applicable to me? I've also applied for adjustment for status and employment authorization under adjustment of status. Will have to pay more taxes once my employment authorization under adjustment of status is approved?

While under F-1 status, you definitely have to pay state and federal income taxes. You are probably exempt from Social Security and Medicare taxes, but that is often a case-by-case decision. If you employer insists on withholding Social Security and Medicare taxes, ask that they be refunded. If the employer refuses, get the refusal in writing, as you can then apply for a refund using Form 483.

Once you convert to a H-1 or H-1B visa, you are liable for Social Security and Medicare taxes and well as the income taxes.