CPO vs CPA
Cost per order is simply the average amount of money a company spends to get anyone to purchase, while cost per acquisition is the average amount of money a company spends to get someone new to purchase.
Here's Sean on when to use each metric and a few things to be aware of when you're using CPO and CPA:
More on CPO and CPA
For an article on CPO, CPA, and ROAS (and to learn what a good CPO is) head over here.