Deputy inspector-general Hassan Salim, is the third company official to be killed in the past four months, police and sources within the company said.
Salim is understood to have been conducting multiple corruption probes inside the company.
He was travelling with three other employees to their office in the northern oil hub of Kirkuk when shooters drove alongside in a speeding car and riddled their minibus with bullets.
No-one has claimed responsibility for the killings and police have not identified the killers.
Kirkuk has been under Kurdish control since June 2014 when Islamic State overran the north of the country and Iraqi security forces collapsed.
Since then there has been a wave of assassinations, many of them targeting Arabs, according to reports.
NOC’s operations chief was killed in late June, followed by its chief engineer Saad Ali Hussain, who was also shot dead.