The German midfielder missed two close-range chances against Manchester City in the first half of the match, which Arsenal eventually won 4-1 on penalties after the game was drawn 1-1.
O'Hara, who was speaking on talkSPORT, said: "Kai Havertz, Arsenal fans can have their say this, but he can't hit a barn door. We know what he's going to give you - he's going to give you hold up play, he's going to work hard, but he's going to let you down when it comes to the crunch. And they have to sort that problem because [Gabriel] Jesus is injured. He picks up too many injuries."
O'Hara's comments have been echoed by some Arsenal fans, who are concerned about Havertz's ability to score goals in front of goal. The 22-year-old has only scored 14 goals in all competitions for Chelsea in the past two seasons, and he has been criticized for his lack of consistency.
However, Havertz's supporters point to his technical proficiency and his ability to create chances for his teammates. He also has a good goalscoring record in the Champions League, having scored 12 goals in 20 appearances for Chelsea.
Only time will tell whether Havertz can be a successful back-up striker for Arsenal. He will have to start converting his chances if he wants to win over the club's fans.