O'Connor points out that her black crop top and high-waisted pants were part of last year's summer clothing line in high street retailer Zara.
It left her "shaking and upset", she wrote. A gent two rows behind me was wearing shorts and a vest top and nothing was said to him.
O'Connor said she had worn the outfit at security check and through the airport and had not been asked to "cover up".
She said that the staff made comments about the incident over the public speaker system.
"I could wear that in Birmingham in the street, and I wouldn't get harassed", she said. "It was the worst experience of my life".
The flier claimed a man also hurled abuse at her while Thomas Crew staff did nothing.
She tweeted: "I was given a jacket by my cousin sitting at the front of the plane and they did not leave until I physically put it on".
An airline has apologised to a 21-year-old woman who alleged that she was directed to put on more clothes or risk being offloaded for wearing "inappropriate" attire, CNN reported on Wednesday. The crew did nothing in response, she said.
She posted on social media about the incident once she was back in the United Kingdom.
"To be honest why did you make all this fuss and not just cover up and get on with the flight", wrote one Twitter user. Or an even bigger question still, why does anybody give a flying (yes I know what I did there) fuck what anybody else wears?
Liz added: "I don't want to criticise you Emily but we live in a world today where we get all these sexy selfies".
Thomas Cook has since apologised to Ms O'Connor, saying "it's clear we could have handled the situation better".
A spokesman for the airline appologized for the incident and said staff 'could have handled the situation better'.
He also added that the airline has an appropriate clothing policy that applies "equally to men and women of all ages" but that applying the policy can be hard and crews "don't always get it right".
For me as a woman, or a man, any gender, any sex, you should be able to show your body in any way you deem appropriate.
Miss O'Connor, who is from Solihull, posted a picture of what she was wearing on Twitter and has since had thousands of messages of support. "Our crews have the hard task of implementing that policy and don't always get it right", the airline added.