O'Neil currently plays for Norwich City, who recently achieved promotion from the Championship to the Premier League, but feels Downing would be a good enough addition to get Middlesbrough to exactly the same position.
Downing enjoyed a superb first half of the season for West Ham last term but has been heavily linked with moves to both Sunderland and Middlesbrough since, most recently by the Daily Mail.
West Ham made marquee signing Dimitri Payet earlier this summer, suggesting they could allow their 30-year-old winger to move on.
And O'Neil feels Downing would be the perfect recruit for Middlesbrough.
Stewy Downing back to Middlesbrough! Wow.. What a great signing that would be in their push for promotion back to the premier league— Gary O'Neil (@Gazoneil) July 5, 2015
Both Middlesbrough and Sunderland are in need of a wide player this summer and the Black Cats' needs may even be greater considering the fact they want the Hammers star to help avoid another relegation-threatened season.
If their local rivals can beat them to a deal, however, there is no reason Downing can't shine at his former club in the Championship. It'll be interesting to see how it all unfolds...