Georgia O'Keeffe's first major New York gallery venue was 291 Gallery, commonly called 291. Alfred Stieglitz ran the gallery at 291 Fifth Avenue, and its address became its name. The space was originally called the Little Galleries of the Photo-Secession before adopting the shorter title.
Stieglitz began showing O'Keeffe's work there in 1916, after receiving examples of her abstract charcoal drawings. These works established her in New York's modernist circle before she had personally visited the city. The exhibitions helped make her one of the most discussed American artists of the period.
291 was important beyond O'Keeffe because it introduced American audiences to European modernists such as Pablo Picasso, Henri Matisse, and Paul Cézanne. The gallery closed in 1917, but Stieglitz later operated other galleries, including the Intimate Gallery and An American Place. Those later venues also exhibited O'Keeffe, so they should not be substituted for 291 in this question.