Arthur Wesley Dow introduced Georgia O'Keeffe to influential modern-art ideas through his teaching.
Dow emphasized composition, line, color, and the arrangement of visual elements rather than requiring students to copy nature exactly. O'Keeffe studied with him at the University of Virginia in 1912, and his approach helped her move away from more conventional academic methods.
This influence mattered because O'Keeffe later developed highly simplified, abstracted forms from flowers, landscapes, bones, and buildings. Dow was not the only important teacher in her education: she also studied at the Art Institute of Chicago and the Art Students League in New York. However, Dow's principles gave her a framework for pursuing an individual modernist language.
Robert Henri and William Merritt Chase were prominent American art teachers associated with other stages of her training, which makes them plausible alternatives. The key connection in this question is Dow's direct influence on O'Keeffe's shift toward abstraction.