Constance O'Day-Flannery is a best-selling American author of romance novels.

Constance O'Day-Flannery has never taken a writing course or attended college. She began writing in 1986 when her son entered school. While reading romance novels during her recovery from a hysterectomy, O'Day-Flannery began to think about the type of book she would want to read. She finished her manuscript 18 months later and sold it quickly. Since then, she has published over twenty novels, all of which have appeared on a national best-seller list. Many of her novels are paranormal or time-travel romances. She has been awarded the Romantic Times BookClub Award for Best Time Travel for Timeswept Lovers and the Romantic Times BookClub Award for Best Contemporary Fantasy Romance, Second Chances.

In 2001, O'Day-Flannery took a hiatus from writing. She spent three years living in Ireland before returning to the United States and continuing her writing career.

O'Day-Flannery currently lives in Pennsylvania. She has two children.