The Blues have agreed a deal with Atletico Madrid for the England midfielder, and it's now up to Gallagher to decide if he wants to leave the club.
However, O'Hara has called the move a 'joke' and accused owner Todd Boehly of making a mess of the club by letting one of Chelsea's academy stars leave Stamford Bridge.
“I think Chelsea selling Conor Gallagher is shocking,” he told Grosvenor Sport.
“Todd Boehly has made a real mess of this football club. He’s come in and tried to make a statement by spending a billion pounds on all these players, but he’s forgotten what the heart is at Chelsea – their academy.
“There is no heart at Chelsea anymore, it’s been ripped out. For me, Gallagher is the heart and soul of the Chelsea team. What he may lack in quality, he makes up for in character and desire to win games – he sets examples for the other young players who are coming into the squad.
“He’s Chelsea’s vice-captain, but they need to get rid of him to balance the books after overspending on the likes of Mikhailo Mudryk and Alex Disasi, who have been average since they came in. It’s an absolute joke.”
Live on DAZN: Terence Crawford vs. Israil Madrimov on August 3. Click here for details.