What is it called when you use an apostrophe to shorten a word? e.g They march o'er scenic hills?
It is called a contraction.
Contractions are usually used to combine two words together (i.e. isn't, couldn't, won't, I've, he'd). However, contractions can also be used to shorten single words. An example of this is "o'er", as you have in you example sentence. Another example is "ma'am", a contraction of the word "madam".
If for whatever reason you need more contractions, here's a webpage full of them.