"After looking at different scenarios, we made the economic decision to end generation in February 2012," said a spokeswoman for Magnox, which is owned by U.S. nuclear services company Energy Solutions.
The 225 megawatt (MW) reactor located in Gloucestershire was granted permission to run until the end of next year, while a twin reactor on the same site shut down for good at the end of June.
Both units were initially scheduled to shut down three years ago, but high safety standards on the site earned them a few more years of operation, Site Director Phil Sprague said.
"Oldbury has provided the UK with a vital source of power for over four decades, something that everyone who has worked at the site, past and present, should be very proud of," Sprague said.
The Oldbury reactor is four years older than Europe's second-oldest reactors at Wylfa in Wales and Santa Maria de Gerona in Spain, which both opened in 1971.
The nuclear site has produced over 130 terawatt-hours (TWh) of electricity since 1967, enough to power 1 million homes for 20 years, and featured in television series such as Doctor Who and Top Of The Pops.
A nuclear new build joint venture between German rivals E.ON and RWE has bought the Oldbury site, as well as a site at Wylfa, as part of its plan to build around 6,000 MW in new nuclear plants in Britain by 2025.
(Reporting by Karolin Schaps; editing by Jane Baird)