Which airline is the primary operator at O'Hare Airport?

United Airlines is the primary operator at O'Hare Airport due to its significant hub operations based there. This means that O'Hare serves as a central point for United Airlines' flights, including both domestic and international routes. As a major airline in the United States, United operates a high volume of flights, utilizing multiple gates and extensive terminal facilities at O'Hare, which contributes to its strong presence.

In addition to the volume of flights, United's strategic investments and expansions at O'Hare highlight its commitment to this airport as a vital part of its operational network. This extensive operation at O'Hare ultimately positions United Airlines as the dominant airline at this key airport, making it the correct answer to the question.