Biden would be the initial frontrunner, but I can see him losing to O'Malley.
I can't see a rock losing to O'Malley in a non-Maryland electorate. He's at 2% in Maryland.
Biden might lead polls at first, and would have a strong chance at the nomination.
But I think the Democratic party wants to nominate someone younger, and they want the milestone of first female President. Kirsten Gilibrand fits that profile, and has conventional qualifications and a record of suitably liberal accomplishments.
These ^^^
I'd add that Klobuchar running as well as Gillibrand is then an issue. Who would become the "replacement" Hillary?