When the Fashion Editor leaves his position at Casual he can no longer go back “to a job where I had to put in a lot of hours,” he told Reuters.
He added: “I’m a designer, not a salesperson, so I’m not a product designer.
So my job is really different to what it used to be.”
Mr O’Sullivan’s departure comes as other outlets, including Vogue and Cosmopolitan, have struggled to find a buyer for their brands.
And it comes as the fashion industry faces new scrutiny about gender-based bias in the fashion business.
In January, the BBC announced it was changing its editorial policy to better reflect the changing gender makeup of the industry.
On Tuesday, Fashion Week said its board was considering revising its approach to gender equity by “identifying a more inclusive and diverse workforce” to better serve women and minorities.