Mr. President.
"Mr. President" is the proper address for the US President. "Mr. Obama" would probably not offend him.
how would u address a letter to the president of the u.s
To address a former president you do not say mr. president or anything having to do with president. <---- This is WRONG. You address any former president as Mr. President. That is a title they have as long as they are alive.
Harry Truman was President all of 1950, 1951 and 1952.
President Bush -- It is customary to address ex-presidents as "president". You surely would not want to address him as just George Bush unless he is a personal friend. Governor Bush or even Mr. Bush would probably do.
mail address/president of France
In newspapers, the President of the United States should be addressed as "President [Last Name]." For example, "President Biden" would be the proper way to address the current U.S. President, Joe Biden.
The letter would find its way to President Putin if it were addressed to Russian President; The Kremlin; Moscow, Russia. The address could be The Kremlin or just Kremlin.
If one were to write a letter to the president, it would be wise to address any issues that are important to that individual. If there was any policy that you disagree with, or any bill you would like to see passed, it would be a good opportunity to address such issues.
You would do it like this: Dear President Obama:
In the United States in 1962, the US president was John F. Kennedy. He would address the Senate or the House if there was a cause to do so and by the Constitution, the president is obligated to address the whole Congress for an annual State of the Nation Address.
In Ireland the correct formal address is "Your excellency" but in practice most people would say Mr./Madam President or President Higgins/McAleese etc