For OCR you will need OpenCV compiled with Tesseract support. See tutorial here, especially using the C++ API: [https://www.learnopencv.com/deep-learning-based-text-recognition-ocr-using-tesseract-and-opencv](https://www.learnopencv.com/deep-learning-based-text-recognition-ocr-using-tesseract-and-opencv)/
I think the deep learning API of OpenCV is only exposed through C++ while the opencv package from Nim (at [https://github.com/dom96/nim-opencv](https://github.com/dom96/nim-opencv)) only exposes the C API so you might need to regenerate your own wrapper (probably [nimterop](https://github.com/nimterop/nimterop) could help)
