The Premier League side slumped to another disappointing result on Saturday, clinging on to a 1-1 draw with Southampton, the third successive time Rangnick's side have squandered an early lead.
United continue to look dysfunctional under the veteran German manager and boos greeted the full-time whistle at Old Trafford, as the Red Devils lost yet more ground in the race for fourth place.
While many have pointed the finger at Rangnick's underperforming players, O'Hara believes the boss is responsible for United's woes.
He believes a 'top manager' must come in when Rangnick's short-term spell ends this summer, with PSG boss Mauricio Pochettino currently the front-runner.
O'Hara told talkSPORT: "For me, Ralf is a complete imposter at the football club. I don't know what this guy is doing there.
"He looks like a school teacher on the sideline. I remember having someone like him manage me at Dartford District, it's that bad.
"The players have seen right through this guy. It's embarrassing to even watch him on the sideline.
"I've never seen anything like it at Manchester United. They're completely flat, completely lost, and the players have got no confidence in the manager.
"It's showing in the pitch, they've got no confidence in themselves, it's a bunch of individuals. Get a proper manager in there, a top manager, and I guarantee you will see better performances and better results."
O'Hara added: "There will always be a couple of players who aren't quite there, in terms of what you expect from Man United.
"But you get the best out of them. That's what your job is as a manager. This guy can't get a tune out of any of them, he's a shambles."