Mary T. O'Hare, 69, chief judge of the Orphans Court of Prince George's County who was active in community and volunteer organizations, died March 13 in the home of a daughter, Ann Payne, in Mount Airy, Md. Mrs. O'Hare lived in Hyattsville.

She was an associate judge of the court for four years before winning election as chief judge in 1970. She was a Democrat. She was vice president of the Ardmore Developmental Retarded Center and had done volunteer work with the American Cancer Society, Salvation Army, Red Cross and the March of Dimes.

Mrs. O'Hare was a member of St. Jerome's Catholic Church in Hyattsville where she had been president of parish ladies of charity, been a teacher and belonged to the sodality. She was active in Girl Scout activities for nearly 40 years, serving as county treasurer for six years and as a troop leader at St. Jerome's.

She was the widow of George O'Hare, a former judge of the Peoples Court of Prince George's County and mayor of Hyattsville, who died in 1965.

Mrs. O'Hare was a past president of the Lawyers' Wives of Prince George's County Bar Association.

She was a native of Washington and a graduate of St. Cecilia's High School and Chestnut Hill College in Philadelphia.

In addition to her daughter, Mrs. O'Hare's survivors include four other daughters, Terry Johnson of Hyattsville, Jean Reichelt of Silver Spring, Patty Wilkenson of Ocean City, Md., and Beth Urban of Mount Airy; three sons, George Jr., of Brookeville, Md., Christopher M., of College Park, and Joseph V., of Scaggsville, Md.; a sister, Theresa K. Madden of Greenbelt, and 14 grandchildren.