Lazenby will depart to look for fresh challenges.
He lead Kent Reliance Building Society for more than 11 years and was a key player in sealing a deal with American private investment firm JC Flowers to invest £50m after the mutual faced severe funding issues.
The mutual transformed into a bank on 1 February and is overseen by parent Kent Reliance Provident Society.
Following Lazenby’s departure, OneSavings will be lead in the interim by finance director Bob Scruton, while the board looks for a successor.
Malcolm McCaig, chairman of OneSavings Bank, said: “Mike and the board have agreed that the time is right for him to step down from his position as chief executive. We are sorry to see Mike go and very grateful for all that he has done and for his role in creating such a strong foundation for future growth.”
Lazenby said: “I have no doubt that OneSavings Bank will go from strength-to-strength whilst retaining its mutual ethos at the heart of the business.”
Meanwhile, OneSavings continues its plan of returning to mortgage lending after withdrawing heavily from the sector when Kent Reliance was hit hard by the credit crisis.
McCaig said it will increase its mortgage lending throughout the year and gradually introduce new, competitive products.
He said: “We do have some targets, but we are being very careful in understanding how the market responds to our return.”
