The actress, who played Sarah-Louise Platt in Coronation Street, welcomed the child at 8am on Sunday at Manchester General Hospital.
O'Brien and her former co-star Ryan Thomas have named their daughter Scarlett Jacqueline Thomas.
According to The Sun, the ex-soap star helped bring on the birth by playing on a trampoline.
"She was desperate to hold her little girl and tried out all the tips to induce the birth," said a source. "Both mother and baby are doing very well."