The 26-year-old actress, best known for playing Sarah Platt on Coronation Street, also spoke of her forthcoming TV projects.
"It was fine. Maxine Peake (O'Brien's co-star) is an amazing actress, and to be honest, it was absolutely no different from kissing a guy. It didn't even feel weird," she told Metro.
O'Brien said that she was "absolutely happy" to be in the show, adding that "it was just a dream come true".
The star, who is soon to begin filming Jimmy McGovern's The Accused, added: "I'm really excited to be a part of it as he's literally the best writer when it comes to drama."