Huh???

You said the company had offered you the job! Either that means that you're their choice, or they're stringing you along.

Any company that makes you an offer and then says "oh, we're still trying to decide between you and other candidates" is NOT A PLACE THAT YOU WANT TO WORK!!!

I hope I've misunderstood what's happened. If they have NOT offered you the job, you do NOT want to tell them in advance that you will definitely take it, BTW, because the offer will not be as attractive.

-- Mark