Suggested Preprocessing methods for OCR on Circular Images
Here is what I did for a client (What you are asking is the same).

Assuming that you have access to certain type of a pattern on the image (or the center of the hole), you could always detect the template to obtain the location of a possible unwarp:



Note that in the transformed image, two region of interests are defined and the region within which we would like to read the characters is the torus lying between two circles. Using this information, we could warp this image using an inverse polar transform to obtain:



I also provide the MATLAB code for this transformation at this file-exchange post.

In the next step, thanks to the template matching, the characters to be read always lie in the local region defined in the image above. Now, if you have appropriate illumination the character segmentation is nothing but a trivial set of image processing operations:

 

The connected components obtained eventually are fed into a neural network or an SVM, first to train and then to classify. This part is composed only of a basic machine learning task, which I won't go into the details, here. 

Don't forget to take good care of the illumination, as it is almost solving the problem half way.