Barry, who hosts a popular podcast Saving Grace, said she is “really excited” to present a series of shows across the year, starting in September.
“You’ll be hearing my favourite songs, we’ll be speaking with some really exciting guests and as always, in true GK Barry style, I promise a lot of humour, chaos and embarrassing stories along the way,” she added.
Marlowe will take over from O’Leary to co-host alongside Vicky Hawkesworth in the 10.30 to 13:00 BST Friday to Sunday.
The popular content creator, who narrated BBC Three’s I Kissed A Girl, previously hosted the Early Breakfast show for a period last summer.
Marlowe said she is “thrilled to be joining the Radio 1 family” and hopes to “captivate your ears, not so much your eyes due to it being radio”.
Presenting duo Danny Mylo and Rosie Madison will take over the Early Breakfast slot from McCullough, who said it was “such a hard decision” to leave after the “most incredible 6 years at Radio 1”.
Northern Irish presenter McCullough, who appeared on I’m a Celebrity…Get Me Out of Here! in 2024, thanked the listeners who have tuned in to him over the years but feels it is “time for a change”.
