The actress's character Sarah Platt was just 13 when she fell pregnant to her classmate Neil Fearns on the ITV1 soap.
Sarah eventually decided to take on the responsibility of motherhood and welcomed her daughter Bethany months later.
"It was really big at the time," O'Brien told PA. "At the time I didn't realise it because I was very new to it all and naive, so I didn't realise what a big sort of splash it made really, but yeah, I did get a lot of people writing in to me, even young girls saying: 'I'm in a similar situation'."
Naming the 2000 plot as her most memorable, she added: "It's because it was the first biggy as well. For me, that really stuck in my head."
O'Brien left Coronation Street in 2007, but recently announced that she would be willing to return in the future.