Answer:
President Obama may be respectfully addressed or referred to as President Obama, Barack Obama, or Mr. President. I am sure the title by which you refer to him would depend on your personal familiarity with him, and the purpose of getting his attention.
Most importantly, the method of address is relevant to the position he's currently occupying - that is, even a boyhood friend should address him as Mr. President, or President Obama, whenever he is acting in the capacity of the Office of the President.
The proper way to address him by reference or in person by anyone not an intimate friend is ALWAYS Mr. President, or President Obama.