The question is a guarantee every time we talk to factories that are about to start measuring their machine efficiency. To us, there are three components to this answer:

First, a good OEE value is higher than it was before. Going from 10% to 15% OEE is excellent. But from 95% to 90% OEE is not. Don't fall into the trap of setting a goal without knowing your baseline; this usually only leads to fiddling with the data until you reach the desired value, and rarely leads to any real improvements. And why not celebrate a low OEE baseline? It indicates that you have much potential to uncover!

Secondly, a good OEE value is broken down into smaller pieces. What good is it to know that you have 60% OEE if you can't tell where the 40% improvement potential is? Make sure to break down OEE into Availability, Performance, Quality (APQ), and then further down into categories and reasons. Using this method, you will know where to focus your resources and making the most impact with your improvement work.

Thirdly, a good OEE value is predictable. Any unanticipated events will result on loss and inefficiency, so the more we can foresee and plan for, the better.   

Learn to love the deviations!