Georgia O'Keeffe primarily used oil paint for her famous flower paintings. Oil allowed her to build saturated color, smooth transitions, and carefully controlled edges across enlarged botanical forms.
Her flower subjects became particularly prominent in the 1920s. By magnifying petals and cropping the composition, O'Keeffe made a single flower occupy much of the picture surface. The approach encouraged viewers to study ordinary natural forms as if they were monumental abstract designs. Although these paintings are often described as abstract, O'Keeffe usually began with recognizable flowers and plants.
Oil was not her only medium. She also produced important works in watercolor, charcoal, pastel, and other materials, especially during earlier stages of her career. The answer refers to the medium most associated with her best-known flower paintings, not to every artwork she made.