The new Finmere Unit at HMP Bullingdon cost £25.3m
Prisons Minister David Hanson has opened a new £25.3m 120-place unit at a jail in Oxfordshire.
The new Finmere Unit - made up of two connected wings - at HMP Bullingdon, will offer a number of facilities.
These include: a first night centre, an induction facility for new prisoners and a detoxification facility for those with severe addiction problems.
In April, The Ministry of Justice told Bullingdon it must take more inmates to ease general prison overcrowding.
But Glyn Travis, of the Prison Officers Association, said at the time the move would lead to "prisoners being locked up in their cells for longer and that would leave them feeling isolated".
Justice Minister Mr Hanson said: "Prison plays a key role in punishing and rehabilitating criminals, and we will always ensure there are places to hold serious offenders who should be behind bars.
"Opening the new Finmere unit is an important step towards our aim to increase capacity of the prison estate to 96,000 by 2014."
The Finmere Unit took 18 months to complete.
HMP Bullingdon operates as an adult male category C training prison with a category B local function.
In April, the Prison Service said the jail's operational capacity currently stood at 994 inmates.