Go to the WorldCat website. Find the search options on the front page.
Click the "Advanced search" link. You will be taken to the advanced search page.
Find the input boxes for entering search terms. There will be three boxes available. Next to each box will be a pull-down menu. Click on the top pull-down menu, which will read "Keyword," and select "Accession number" from the list of options.
Input your OCLC number into the text box next to the pull-down menu. Click the "Search" button. If your OCLC number is correct, you will be taken to your desired publication's record in WorldCat.
Scroll down to the "Details" section of the page.
Find the row marked "ISBN." The numbers in this row are the ISBNs for this publication. Most publications have more than one ISBN. Each one is separated by a space.