The Black Cats manager is aware the upcoming transfer window will be his "only opportunity" to make addition, in an attempt to push his side above 15th place in the Premier League
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.
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"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."