Just days after leading Spurs to the Europa League title, Postecoglou was axed by chairman Daniel Levy and eventually replaced by Thomas Frank, formerly of Brentford.
Asked about the decision, O'Hara told Tribalfootball.com: "Yeah, the reason (I agree) is I watched them all season. You won the Europa League. Well done. Like, he got that monkey off his back. You know, he put all his eggs in that basket. Let's be honest. The teams that we played weren't that great, right?
"Bodo Glimt, AZ Alkmaar, Eintracht Frankfurt and a terrible Man United team. They're the teams.
"The games that we won with Frankfurt, we were terrible. Bodo Glimt, we played all right. AZ Alkmaar, we were terrible. Man United, we were terrible. We won it. Great. You know, you've got it.
"But the Premier League, when you look at it, it's your bread and butter. You can't lose 22 games as a Tottenham Hotspur manager. They're meant to be a big football club. Commercially, globally, Tottenham can't lose 22 games. It's unacceptable."
Ange fluked Europa League title
O'Hara insists Postecoglou should not have focused solely on the Europa League.
He continued: "And I don't care, if you say I want to put all my eggs in that basket. Even still, I don't think that Daniel Levy looked at it and thought, 'I'm going to see a massive improvement. I'm not going to see a top five team next season under Ang. So I've got to make a change'.
"And look, people might say it's harsh because he finished fifth in his first season and he won a trophy in his second season. You might say it's harsh, but I just don't think Daniel Levy saw the future with him.
"In a way, you can say he fluked the Europa League, do you know what I mean? Because of the teams he played. But 22 losses, finishing 17th. You can't finish 17th. It's unacceptable."
- Jamie O'Hara was speaking to Tribalfootball on behalf of Grosvenor Sport