The move means that fellow joint managing director Tim Bleakley will take the newly created role of UK managing director for sales and marketing. In addition to his UK duties, Bleakley will also take overall responsibility for international sales.
Tom Goddard, chief executive of Viacom Outdoor, said: "I am delighted to announce Andrew's promotion to chief operating officer. With nine years' experience in outdoor, Andrew has a proven track record of management."
Oldham said: "We are entering a period of intense competition for contracts, for advertising revenue and for share versus other media. I think we have a fantastic team in both countries, and I am proud to take on this new role."
The Republic of Ireland senior management team and their Northern Ireland counterparts will all report to Oldham.
The international division has also been reshuffled, with the appointment of Clive Punter, currently the chief operating officer of Viacom Outdoor International, taking the role of chief operating officer for Asia. Punter is being seconded to China to oversee the company's existing Asian business during the lead-up to the Beijing Olympics in 2008 and to head the company's expansion plan across the Asian region.
Guillaume Prot, currently Punter's opposite number as co-chief operating officer at Viacom Outdoor International, moves to become chief operating officer for continental Europe.
He will be responsible for leading the businesses in France, Italy, the Netherlands and Spain. The local managing directors in each of those countries will report to Prot.
As part of the changes, Mark Pettler, currently delivery directorat Viacom Outdoor UK, takes on an expanded international role, being promoted to international operations director. He will help coordinate best practice in products, innovations, procurement, operational processes and logistics.
In May, the company retained the £800m London Underground advertising contract.
Viacom Outdoor will change its name to CBS Outdoor in the UK on January 1.
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