Jerry Petherick, managing director for G4S Custodial and Detention Services, said: "The mobilisation of any prison is a complex and challenging operation but the size and scale of Oakwood - the largest prison in the country - made this even more acute.
"As well as the logistical hurdles in new establishments, prisoners test the regime as well as the members of staff, who may be new to prison life.
"We have already taken steps to make improvements, appointing a dedicated task-force to address problem areas, such as the prevalence of drugs, while providing additional funding where necessary. This is starting to yield results."
More top news
21 people died after bombs exploded in two Birmingham pubs in November 1974. Can you help us tell their story?
Julie was 11 when she lost her big sister but has spent her life seeking justice for Maxine and the other 20 victims of the pub bombings.
Brian Davis believes his sister Jane would have gone on to great things if her life hadn’t been cut short in the Birmingham pub bombings.