Anacrusis

Anacrusis in poetry is the lead-in syllables that precede the first full measure, while, similarly, in music, it is the note or notes (even a phrase) which precede the first downbeat in a group. The latter sense is synonymous with upbeat and is often called the pickup or the pickup note.

In the Star Spangled Banner, the word Oh in the first line is in anacrusis in both the music and the anapestic meter of the poem:

x    /    x   x   /    x   x   /     x  x  /   
Oh, say, can you see, by the dawn's early light. . .