Oscar, whose surname has not been released, went missing from the Upper Arncott area of Bicester.
The 15-year-old was last seen on Sunday, September 13 and is believed to still be in the Bicester area.
Thames Valley Police are asking for help in locating a missing boy and have released his image and description.
Oscar is described as a white male with a thin build, pale skin and red hair shaved at the bottom, possibly wearing a dark tracksuit.
A Thames Valley Police spokesman said: “We are concerned for Oscar’s welfare. If you have any relevant information or have seen him, please call us on 101 quoting reference 43260479292.”