O'er is a poetic shortening of the word “over.” Poets use it to make their lines flow better and fit the rhythm they want. You'll find it in old poems and songs, like when Francis Scott Key wrote “o'er the land of the free” in “The Star-Spangled Banner,” meaning the flag was flying over the land.

Writers shorten “over” to “o'er” for the same reason musicians sometimes hold a note longer or shorter: to make the rhythm work. If a line of poetry needs exactly ten syllables, “o'er” (one syllable) fits better than “over” (two syllables). The apostrophe shows where letters were removed.

You might also see it in Christmas carols like “Angels We Have Heard on High,” which describes “echoes of their joyful strains o'er all the plains,” meaning the angels' songs rang out over the fields.

People don't use o'er in regular writing or conversation today. It belongs to poetry, songs, and older literature. When you see it, just read it as “over” and you'll understand the meaning perfectly.