I think your OPT will be extended automatically when you file H1B petition before expiration of your F1 status. Since you applied during the valid OPT period (not in the 60-day grace period), you may continue working till 30th September 2012. Hence you don't have to extend your OPT explicitly.
Read the "Regulations Relating to F-1/H-1B Cap-Gap" section in the USCIS website: Questions and Answers: Extension of Optional Practical Training Program for Qualified Students