It's a good idea to run OCR on the scanned PDF BEFORE you start adding a table of contents (TOC) to the PDF. This is because OCR apps usually create a new PDF, and may not know how to preserve the table of contents from the original PDF when writing out the new PDF. Your hard work in creating the TOC would then be wasted. Moreover, PDFs with OCR text allow text to be selected which can be used to build the TOC quickly as described below.
Note that PDFOutliner comes with an automatic TOC feature which parses the fonts used in a PDF - such as larger fonts used for chapter headings - to build a TOC hierarchy. However this "AutoTOC" feature doesn't work on a scanned PDF (which contain no text) or even an OCR PDF (which do not assign different fonts to chapters/body text etc.) In such cases, you have to "walk" through the PDF page by page, and add TOC entries. Here's how: