The Black Cats sit 15th in the Premier League before the Boxing Day fixtures and are one of only three teams in the division to have scored fewer than 20 goals.
"It's the only opportunity that you're going to get," O'Neill told reporters of potential additions.
|13/2||Sunderland are 13/2 with Bet365 to draw 1-1 with Manchester City|
"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."