Thomason said Walter Sayles called police at about 5 a.m. Wednesday to report a shooting in the 9200 block of Cherry Street.
He said officers met with Sayles but no evidence or crime scene could be located.
Sayles wasn't hit by gunfire but he was transported to Highland Hospital in Oakland to be treated for an unrelated injury, Thomason said.
Sayles asked to speak with investigators about the shooting, and the reporting officer told him to go to the criminal investigation division at police headquarters in downtown Oakland after he left the hospital, Thomason said.
The reporting officer then advised investigators at police headquarters that Sayles matched the suspect description from a recent armed robbery, according to Thomason.
After investigators talked to Sayles about the shooting, they asked him about recent robberies, Thomason said.
He said Sayles then confessed to committing more than 30 armed robberies at Quik Stop Markets and 76 and ARCO gas stations in East and West Oakland.
Oakland police are working with neighboring agencies to sort out and identify additional robberies in the Bay Area in which Sayles may also have been involved, Thomason said.
He said the investigation into the robberies is ongoing and anyone with information should call the Oakland Police Department's robbery and assault unit at (510) 238-3326.