A dog walker got "a bit of a surprise" when he found an 8ft (2.4m) boa constrictor in a field, the RSPCA said.
The animal welfare charity was called on Saturday by a member of the public who found the 12.2kg snake near a housing estate in Banbury.
The charity said it was not known how it came to be in the field.
Deputy Ch Insp Melanie Fisher said the reptile was now doing well despite the cold weather.
She added: "I think the caller got a bit of a surprise. It's not every day you stumble upon an 8ft snake while out walking the dog.
"The snake may have escaped from a vivarium or may have been abandoned. Sadly she's not micro-chipped so we do not know where she has come from."
The owner is urged to get in touch with the RSPCA by calling its inspectorate appeal line.