The Black Cats find themselves just two points above the drop zone after Saturday's 4-2 home defeat against West Brom.
O'Neill feels new additions are needed if Sunderland are to avoid finding themselves involved in a relegation battle this season.
"There are so many games between now and the New Year and my focus is on those games more than anything else," said O'Neill.
"Will I do much in January? I have mentioned before that we have less physicality than most Premier League teams.
"I do think when we get momentum that we should thrust forward, rather than falling back again, which allows opposition to get back into line."