David Boren is an interesting guy. A Conservative/Centrist Democrat from Oklahoma who Barry Goldwater praised as somebody who ought to be president, Obama had on his national security team, Perot wanted him as a running mate in 1996 and Bloomberg contemplated as somebody who'd be a good running mate in 2008.
How could Boren become President?
I think the easiest way is for him to be picked as somebody's Vice President. Clinton considered making him defense Secretary in 1994 and Gore in 2000 wanted somebody tough on defense as his running mate, so perhaps Gore-Boren could work?
Alternatively Boren might work as Obama's running mate (he served on his intelligence advisory board in the first term).