United and American Airlines both offer $83 one way fares from San Francisco to Chicago ORD. More dates are priced at $89 each way in April and May.
United Airlines has more low fare dates than American Airlines.
You can even fly United Airlines Chicago to San Francisco for $64 one way on April 28 to create a $152 round trip ticket.
United Airlines Chicago ORD – San Francisco SFO (nonstop flights)
$151.20 round trip Thursday April 28 – Saturday May 7
American Airlines San Francisco SFO – Chicago ORD (nonstop)
$181.20 round trip Saturday April 30 – Saturday May 7
SFO-ORD-SFO = 3,692 flight miles for AAdvantage members.