President Obama has re-taken the Presidential Oath, merely because he'd said the word "faithfully" in the wrong place (though still correctly, in grammatical terms). But it matters, because the wording is precisely set by the Constitution. And what a serious oath it is:
"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."
The Vice-President's oath is set by Congress, and in its latest (1884) wording is even more determined to leave no room for lawyerly ratting-out:
“I 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.”
A foreigner who wishes to become a US citizen must say:
"I hereby declare, on oath, that I absolutely and entirely renounce and abjure all allegiance and fidelity to any foreign prince, potentate, state, or sovereignty of whom or which I have heretofore been a subject or citizen; that I will support and defend the Constitution and laws of the United States of America against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I will bear arms on behalf of the United States when required by the law; that I will perform noncombatant service in the Armed Forces of the United States when required by the law; that I will perform work of national importance under civilian direction when required by the law; and that I take this obligation freely without any mental reservation or purpose of evasion; so help me God."
My brother took this oath last year, and knowing him, will have meant it and will uphold it down to the last punctuation mark.
Compare that with the British version, with its room for legal manoeuvre and evasion:
"I (name) swear by Almighty God (or, “do solemnly, sincerely and truly declare and affirm”) that on becoming a British citizen, I will be faithful and bear true allegiance to Her Majesty Queen Elizabeth the Second, her Heirs and Successors, according to law."
I could wish that all our holders of political office in the UK would be compelled to use the American wording, substituting only "Kingdom" for "States of America".
And we appear to have forgotten (and compare it with the current US citizenship oath above!) the following extract from the Oath required by the 1689 Bill of Rights Act:
"I do declare that no foreign prince, person, prelate, state or potentate hath or ought to have any jurisdiction, power, superiority, pre-eminence or authority, ecclesiastical or spiritual, within this realm."
George III, whose actions were part of the causes of the American Revolution, knew the power of his Coronation Oath: “Where is the power on earth to absolve me from the observance of every sentence of that Oath? [...] I can give up my crown and retire from power. I can quit my palace and live in a cottage. I can lay my head on a block and lose my life, but I cannot break my Oath. If I violate that Oath, I am no longer legal Sovereign in this country.”
The Monarch's Coronation Oath has been amended (e.g. in 1937 - removing a vital reference to the "law and Customs" of the Kingdom) since then; and although the Common Law is enshrined in the 1701 Act of Settlement, it may have to give place to Europe's directives and such rights as its legislators are minded to grant (and, presumably, amend, suspend or withdraw).
In some senses, Britain is a younger country than the United States of America; and perhaps the worse for it; for here, I fear, one's word, oath, anciently (I believe) one's gesa, and the law itself, have become sandy; not a rock on which to build.