Because I heard this year's quota being used really fast, my opt ends at the end of September, I just want to make sure that I could get my H-1B before that. Although it will begin to be effective on Octorber. So even my H-1B gets approved now, I will still wait until Octorber 1st to work using H-1B.

As for the social security and medicare tax, I have been an F1 for more than 5 years, so need to pay these things even during OPT. That's what I heard.