A-okay was used rather than okay during the space program for radio communications. The reason for this is that the "A" sound gives a short hard sound at the start of the word as opposed to the longer more drawn out sound of the "O". This meant that the listener was much less likely to miss the word in amongst the background noise common with radios in those days (particularly when the other person is on the opposite side of the ionosphere).

Thanks to Tem42 for the correction.

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