Cavani has been a hit with eight goals and two assists since his arrival at Old Trafford on a free in the summer.
The 34-year-old has been strongly linked with a return to South America once his one-year deal at United ends.
But O’Hara says it would be great business from United if they extended his stay as he hailed the Uruguayan.
The former Tottenham man even claimed Cavani would have been considered one of the Premier League’s best-ever strikers had he moved to England a few years earlier.
“If he came and played in the Premier League four or five years ago when he was in his peak, we would be talking about one of the best strikers to EVER play in the Premier League,” he said on Monday’s talkSPORT Breakfast.
“We’re getting him at the end of his best, but he’s still producing magical moments.
“It’s not going to be for another two or three seasons, but as a striker he is phenomenal.
“If you’re a young striker and you want to make it in the game, you need to watch Cavani religiously.
“His movement is brilliant, it’s incredible. He always plays on the shoulder and it’s so hard to defend against, he’s so hard to mark.
“He’s been a great signing for Manchester United and I think they should try and keep him, I really do.”