The Black Cats sit 15th on the Premier League table having scored just 19 goals in 18 matches and O'Neill is aware he needs to bring in some new faces in the winter window in order to improve that output.
"It's the only opportunity that you're going to get," O'Neill told reporters.
"Once that window closes, that's it, then, until the end of the season so it's the one opportunity you have to try and see if you can strengthen your side.
"Easier said than done, of course, like everything else, and sometimes the prices become a bit prohibitive.
"But, that taken into consideration, I think we have to be looking at it just in terms of strength in depth."