I, (name), do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter. So help me God.
I'm not sure what, if any, oath a contractor might take.
I am a contractor, I did not take this oath.
If nothing else, a contractor with a security clearance will sign documents wherein he agrees not to disclose classified information to unauthorized persons under penalty of heavy fines and imprisonment.