OCR works best for black on white or binary images, where letters have sharp contours, no noise, and solid filling, i.e. they are nothing like this:
For the curious it's a part of a document number that is embossed. Applying unsharp mask, thresholding, closing and median operations already improves it a bit, but it still cannot be processed properly by OCR.
What other operations could bring the image to the preferable state like this manually edited one: