Fire crews were called to Wiggington Road, South Newington, at about 10:20 GMT.
The horse, named Tight Squeeze, fell unconscious after becoming stuck in the deeply banked ditch for several hours.
Fire crews and the service's animal rescue team used slings and harnesses to pull the 18-year-old from the ditch.
Oxfordshire Fire and Rescue Service said the rescue became "more complex as she was sinking further into the mud in a deeply banked, extremely muddy swamp-like ditch".
The service said following the rescue the race horse slowly raised herself and stumbled on to her feet.
A former racehorse has been rescued by firefighters after slipping into deep mud in a ditch in Oxfordshire.