Oldstein joined MetLife in February as an interim appointment in the newly created role and has now joined as a permanent member of the team.
She is responsible for all aspects of marketing communications, including advertising, public relations and internal communications.
MetLife says she played a major role in the June launch of its Retirement Portfolio.
Oldstein previously worked for Post Office Financial Services, a joint-venture between the Post Office and Bank of Ireland, where she was a founding member.
She held several roles on the executive team at POFS, including head of product development, head of marketing and communications director.
MetLife UK chief executive officer Ed Gardner says: “Our plans are for continued growth and Claire will play a key role in promoting the value of our proposition to the UK market.”
Oldstein adds: “MetLife has major ambitions in the UK and I am delighted to be confirming my appointment with the company at such an exciting time.”