The teenager was reported missing after 8pm on 2nd March. She was last seen outside Abingdon College on Wooton Road in Abingdon at around 3.30pm that day.
Detective Inspector Kev Elkins said: “We are growing increasingly concerned for Courtney’s welfare and I would urge anyone who knows of her whereabouts, or who has seen her, to come forward and speak to us as soon as possible.
“I would also like to appeal to Courtney to come forward and make contact with us and let us know that she is safe and well. We believe she was in the area around Posh Nosh in Abingdon at approximately 5.30pm on Friday. Did you see her? Do you know where she went?"
Anyone with information is being asked to contact Sergeant Jason Shillam using the 24-hour Police Enquiry Centre on 101 quoting the reference number MP/01447/12.