Dr Ubani, registered by NHS Cornwall and Isles of Scilly, gave 10 times the recommended dose of diamorphine to a patient who later died in February 2008.
In a parliamentary debate last week, Mr O'Brien said some PCTs had not carried out the 'tests and checks they are legally obliged to undertake on a person's ability to speak English'.
He added: 'I understand that Cornwall was not carrying out adequate checks but I am told it is now.'
A problem with the PCT was raised in an earlier debate in 2007. Truro and St Austell Liberal Democrat MP Matthew Taylor told MPs that a patient reported an overseas agency doctor 'who not only found it difficult to understand her but relied on an electronic word converter to communicate'.
Shadow Conservative health minister Mark Simmonds added that 'rumours are circulating that one or two PCTs have a reputation for giving easier access to the performers list'.
Mr O'Brien said he expected an out-of-hours review by the DoH and RCGP to call for more action 'to ensure that PCTs perform their control obligations'.
NHS Cornwall and Isles of Scilly was unavailable to comment.