The Chicago-based airline's CEO made the announcement at the airport on Wednesday.
The fuel, also known as SAF, is made from renewable or waste materials.
The fuel can emit up to 85% less carbon on lifecycle basis, compared to jet fuel.
United said it used more SAF in 2023 than any American airline at airports in California and Europe.
Governor JB Pritzker was in attendance at the announcement to show his support for the initiative.
The Illinois SAF tax credit was signed into law in 2023 to provide framework for the airline to begin purchasing SAF for use at O'Hare.