Cooper has stepped down from the board with immediate effect though will remain an employee of the company during a short handover period.
Alan Booth, a current non-executive director, has been named interim chief executive and he will hold that position until a new chief executive is hired.
Ophir said that the decision reflects an ongoing move to rebalance its portfolio towards larger production and cash flow base, to support more focussed and sustainable exploration activity.
“Today, we have a production-focused business model, exemplified by the proposed Santos transaction. This positions the company well for the future and provides a great platform to Nick's successor,” said chairman Bill Schrader, Ophir chairman.
Nick Cooper added: “I have thoroughly enjoyed my seven years at Ophir and am proud of what we have achieved together.
“Ophir has transformed from a high risk, frontier explorer to a sustainable, balanced, upstream company.
“A solid platform has been built for new leadership to take the company forward. I wish the staff, board and shareholders of Ophir all the very best for the future."
Ophir shares were down 3p or 4.56% changing hands at 62.8p.