Mary ONeill was raised in what she describes as a wonderful barn of a Victorian house in Berea, Ohio, where she wrote and directed plays for her younger brothers and sisters . She was educated at Our Lady of Lourdes Academy, and Western Reserve in Cleveland and the University of Michigan. Mrs. ONeill entered the advertising field and became a partner in her own advertising agency. She retired from advertising and lived in New York City.

Mrs, ONeill's books include the very popular books of verse Hailstones and Halibut Bones and People I'd Like to Keep, and the distinguished Saints: Adventures in Courage.

Mrs. O'Neill died of heart failure on January 2, 1990, in Yuma (Ariz.) Regional Medical Center. She was 84 years old.

One of her most popular books, ''Hailstones and Halibut Bones: Adventures in Color,'' illustrated by Leonard Weisgard, was published by Doubleday in 1961 and printed in several languages. In it, Mrs. O'Neill identified and interpreted the most familiar colors through verse. The book was used as a supplementary text in schools in the United States and abroad.

Other books for which she wrote the text include ''The White Palace,'' about a Columbia River salmon; ''People I'd Like to Keep,'' a collection of brief sketches of saints, scholars, soldiers and kings; ''Words Words Words,'' and ''Take a Number.''

Born in New York City, Mrs. O'Neill attended Western Reserve University and the University of Michigan. She was an advertising copywriter in Cleveland before becoming a freelance writer.

After the death of her husband, John, she joined the Peace Corps in 1970 and taught journalism and writing for several years in Ghana and Costa Rica.

She is survived by two daughters, Erin Gibbons Baroni, of Wyckoff, N.J., and Manhattan, and Dr. Abigail Hagler, of Yuma.; a stepson, John R. O'Neill, of Montgomery, Ala., and four grandchildren.