[mysterio]

So does that mean that everything up to the oath, such as executive orders, have to be reissued?

No. The 20th Amendment made him pres at noon on the 20th, oath or no. And according to what I've read, some Presidents in the past just said "I do" and didn't repeat the oath at all.

[cmj328]

The 20th Amendment made him pres at noon on the 20th, oath or no.

He is President at noon but he can't execute the office before taking the oath. Here, bookmark this site and become a better citizen: constitution.org.

Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation: — "I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States."

some Presidents in the past just said "I do"

"I do" is sufficient if the oath is posed to him correctly. But the oath was posed incorrectly, and Obama didn't say "I do".