For example, you OPT is from 07/01/2005 to 07/01/2006. You receive a offer which will start on 10/01/2005, assumed that there is still H1b quota, you can apply H1b at 10/01/2005 while you are holding your OPT. You just put the starting date on 07/02/2006 right after your OPT expires.

Trust me, apply H1b as soon as possible while your employer agrees to petition you. Don't wait till your OPT expires. You can still enjoy your OPT even your H1b gets approval. It is double security.