What is the possibility to reenter in US

Thanks for the information. If we get the approval of H1 b than our
previous f1 status void!

Because, I was trying to go US but from London they send back to
India. The reason is that they told me that your h1 got approved and
you are no more student. So, you can not enter with OPT. So, in that
case can I extend my student visa(which was expired on 16th July
2012).

I am asking this question because I have a job and currently I am
working through that employer. So, what is my option either I can
applied for extension of f1 visa or wait until 21st September! Because
right now I can not work from India due to h1 approval(h1 start on
OCT). If I can go there than i can probably work over there!

http://redbus2us.com/process-of-f1-to-h1b-visa-change-of-status-opt-rules-planning-tips/#comment-48285

Yopu cannot extend your F1 as you have graduated and on OPT. F1 visa is extended only when you need more time to finish your degree, it simple logic why would they give you student visa when you no longer want to study. Your should talk to your school to confirm the same. Ultimately you can only start working on H1b from Oct.