The Olympics almost never gets the national anthems "right," in the sense of having them sound the way people of the respective nations expect them to sound. It would be much easier for the organizers to simply go to the respective nations' teams and have them provide the sheet music and/or recordings for their national anthems, but that doesn't seem to have occurred to anyone in charge.
Having said all that, Anacreon, the tune that eventually becomes the tune for our national anthem, was originally written in C:
But as with everything else in history, this is not all that it seems. The "C" of the late 1700s was tuned to a lower pitch than the "C" of today, so someone singing a "C" in the late 1700s would be singing a tone somewhere around the "B" below today's C. Moreover, "To Anacreon in Heaven" was a drinking song, and people who are drinking can't effectively control their vocal cords, so someone who is SUI (singing under the influence) is probably going to sing flat. Put all that together, and chances are that the people singing Anacreon in the late 1700s might have thought of themselves as singing in C major, but they might well have been singing at a pitch level equivalent to Bb major by today's standards.