Mark, aged 72, from Little Wittenham, near Abingdon, was last seen at his home at 10.30pm on 23 February.
He is about 6ft 2ins tall, has grey hair and wears glasses pictured below. He may be wearing a navy blue jumper, navy blue waterproof trousers and brown shoes.
Detective Chief Inspector Dave Horsburgh, based at Abingdon police station, said: “We are growing more and more concerned for Mark’s welfare as he has now been missing for more than two weeks.
“We are re-appealing for the public to help us locate him. We would ask the public to be vigilant around the areas of Little Wittenham village and Wittenham Clumps.
“I would urge anyone who may have seen Mark, or anyone who thinks they know where he is, to please get in touch by calling 101, quoting reference 43220083862.”
