Final answer:
The twice-used contraction in our national anthem is 'O'er', which is a shortened form of the word 'over'.
Step-by-step explanation:
The twice-used contraction in our national anthem is 'O'er'. The contraction 'o'er' is a shortened form of the word 'over'. It is commonly used in poetry and songs to maintain the rhythm and flow of the lyrics. In our national anthem, 'o'er' is used to mean 'over', emphasizing the enduring and proud nature of the star-spangled banner.