Screech has been working at Proximity on a freelance basis for the past few months. She will take up the new post immediately, although O'Regan is working out his notice period.
After six years as data director at Joshua, Screech had a short-lived stint as managing director of marketing agency Ruby Mountain from October 2004.
She said data needs to be at the heart of Proximity's thinking, providing a platform for distinctive and effective creativity. She will manage a team of 35 dedicated data analysts working across clients such as Volkswagen, O2 and BBC TV Licensing.
O'Regan is believed to be joining a WPP agency. He has been with Proximity since it was created in 2001 by the merger of Clark Hooper and BHWG, which he had joined in 1995.
Chris Thomas, Proximity chief executive, praised the "tremendous job" done by O'Regan during his time at the agency.
"With change however, there comes opportunity. We have a fantastic team at Proximity and Julie will bring fresh energy and new perspectives in her leadership role," he said.
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