It's safe to say the former Tottenham and Wolves midfielder was no fan of pre-season training during his playing career and is stunned at current players involved in what he called a 'pre-pre season'.
Ronaldo is on holiday with his family, but is staying fit and has posted pictures of his ripped physique to his millions of social media followers, while teammates Scott McTominay and Axel Tuanzebe have also used their downtime to head to the gym.
"I hated pre-season, I hated it with a passion," O'Hara told the talkSPORT Breakfast show.
"Off season is off season, go away, enjoy yourself, put on a bit of weight, come back. Pre season is to get fit.
"Now the players don't work enough during the season and do a 'pre pre-season' to come back fit. I'm like, 'just work harder in the season and relax when you're off, mate. Enjoy yourself'.
"Pre season is to get fit, these lads are staying fit."
O'Hara also revealed he was sent to 'fat club' for being beaten in a pre-season bleep test by goalkeeper Alan Julien, who he spent three years alongside at Billericay Town.
"He was older than me as well. He was fit as a fiddle," O' Hara joked.
"It's an unwritten rule, every footballer knows this; you can't get beat by a goalkeeper in the fitness runs, it's just unacceptable. Now, these 'keepers are fit."
When asked if there was a punishment for his bleep test failure, he replied: "Yeah, I got put in fat club.
"I had to come in an hour before everyone else for fat club."
O'Hara, though, is fitter in his post-playing career and was recently training for a celebrity boxing match, also showing off his ripped physique like Ronaldo.
It appears that Manchester United players sharing how good they look ahead of the new season is becoming a trend this summer.
Tuanzebe was sent out on loan twice last season to Aston Villa and Napoli and he will be hoping to start off on the right foot with Erik ten Hag now at the helm.
Scotland international McTominay made 37 appearances last season and with the exits of midfielders Paul Pogba and Nemanja Matic, the 25-year-old will be staking a claim for a starting berth next season.
Despite links with a move away, Ten Hag has already confirmed that he will be relying on Ronaldo's goals next term as United begin their rebuild.
The new manager took his first session at United on Monday and the players who were not on international duty joined him, while he could link up with Barcelona's Frenkie de Jong before the new campaign begins in August.