The oath to be taken by the president on first entering office is specified in Article II, Section 1, of the Constitution:

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.

This was followed, at least this time, as President Barack Obama entered into his second term, "So Help Me God."

Lord, you heard this solemn oath uttered before witnesses by Barack Obama. I ask you to help him to to keep it with a true heart and to use the power of the Office of the President of the United States, which you have given him, to do as he has vowed. I further ask that if he betrays this oath that you would not hold him guiltless. Lord Jesus, you have accomplished many good things around the world with this nation the last few hundred years and I ask that you do not let any one man, nor organization, destroy it before its time. American has the blood of many babies on our hands because of Abortion and I ask for mercy and forgiveness, but only if we, as a nation, repent. Otherwise let your righteous judgements fall upon the unjust of this world and bring the fear of the Lord upon America... The ungodly brag about the advent and progress of homosexuality and I ask that you let those ungodly ideals and words, that they are so PROUD OF, become as rotted flesh in their mouths. Israel complained and bemoaned about wanting meat and you gave them what they cried for and it rotted in their mouths. So do I ask the same with the ungodly bragging about their ungodliness. Let it becomes as rotten flesh in their mouths and cause them to vomit and gag upon their lusts. Lord, and unless our President repents, start the gagging right there so the whole world can see that there is still a God who rules and puts up who he wills and takes down who he wills. You STILL have all power in Heaven and in Earth. In Jesus Name I pray.