United Airlines has a low $63 fare available for one-way nonstop flights between San Francisco and Chicago for travel from December into February. There are about 25 dates with low one way fares from SFO-ORD at $63 with some $73 one way fares in November during the week before Thanksgiving. There are fewer dates, I count 17, with $63 fares for ORD-SFO segment between December and February, and about a dozen more dates with one way fares from $73 to $92.
Round trip is as low as $126.20 to $155 for SFO-ORD or ORD-SFO with United Airlines.
American Airlines has a few dates with $63 one-way fares on this route, but only for a few Nov-Dec dates, and far fewer low fare dates than United Airlines.
United Airlines Chicago ORD – San Francisco SFO
$126.20 Saturday January 16 – Thursday January 21
United Airlines San Francisco SFO – Chicago ORD
$126.20 Saturday Dec 5 – Tuesday December 8