O'Hara and the director collaborated on five films, including the St. Patrick's Day classic "The Quiet Man."
Recalling the incident on the set of the 1955 film, "The Long Gray Line," the 83-year-old screen legend writes: "I walked into his office without knocking and could hardly believe my eyes. Ford had his arms around another man and was kissing him. I was shocked and speechless. I quickly dropped the sketches on the floor, then knelt down to pick them up ...
"They were on opposite sides of the room in a flash," she said.
Identifying the man with Ford only as "one of the most famous leading men in the picture business," O'Hara said he later approached her and asked her why she had never mentioned Ford was gay.
"I answered, 'How could I tell you something I knew nothing about?'"
"'Tis Herself," which spans O'Hara's life, focuses mainly on her work during the Golden Age of Hollywood. The book is on sale now.