The Black Cats have to date added only free agent Carlos Cuellar and 34-year-old striker Louis Saha to their squad since the end of last season with just 10 days to go until the deadline.
However, amid a whirlwind of speculation over who might end up at the Stadium of Light and with O'Neill having proclaimed himself ready to launch a late flurry of activity, Gardner is confident his former Aston Villa manager will get the men he wants.
He told the Sunderland Echo: "I've worked with the gaffer before and he's not a genius for no reason.
"He's not forced to buy players. He hasn't won medals and been as successful as he is by rushing in and people telling him what to do.
"He decides what needs to be done, and that's what makes him so good."
O'Neill, with the backing of owner and chairman Ellis Short, has made a number of approaches for big-name players, and while progress has been made, he is still awaiting decisive breakthroughs on several fronts.
His pursuit of Wolves striker Steven Fletcher has been well-documented with the Black County club holding out for the best price possible.
Sunderland are understood to have offered a deal amounting to ?13million which is yet to be accepted, although there is a growing feeling on Wearside that the Midlands' stance is gradually softening and that a deal could be done before the end of the month.
Their interest in Manchester City midfielder Adam Johnson is genuine and ongoing, but while the fact that the England international has north-east roots is in their favour, they face competition from clubs with higher profiles.
O'Neill, however, has back-up plans as well as additional targets in other areas of the field, and consequently has seen himself linked with a growing number of players, with reports on Tuesday alone presenting a list of names - some more realistic than others - which extended into double figures.
But there is little doubt that his main focus, as it has been all summer, remains on attacking recruits following the loss of Asamoah Gyan and Nicklas Bendtner from an already depleted armoury.
Fletcher and Johnson would certainly fit the bill, and while the dream scenario would see both arrive, there is ground still to be covered and a succession of obstacles in O'Neill's path.
In the meantime, Gardner is convinced that the Ulsterman has already strengthened his hand in the striking department with the capture of Saha, who made his Barclays Premier League debut for the club as a substitute in Saturday's 0-0 draw at Arsenal, on a one-year deal.
"When Louis came on, I thought he was unbelievable. He held the ball up, was strong and that's what we needed," he said.
"Even in the changing room, it helps when you see someone who has been at Manchester United, and he's a top-class player and top-class person.
"He talks to you, tells you and encourages you, and that's what you need. If we can get a few more players in to go with his experience, that would be great."
