In Mosaic click on New Window button and look for Open Interest By Strike. In Classic TWS click on Analytical tools menu and look for Open Interest By Strike. Click on the (Configure) gear icon on top-right to select mode Option Volume or Open Interest (might not be available for non-US underlyings) and chart Put&Calls or Put+Calls.
Find more information on the Option Activity window in TWS Users' Guide.
You can also add the "Put/Call Interest" and "Put/Call Volume" columns to a watchlist or a market scanner.
Additional Information