The hire is part of a drive by Cubitt to expand its investment and retail financial services offering.
O’Sullivan will be involved in business development and providing clients with strategic advice and media relations services.
He joined Fleishman-Hillard in 2010 as an account director, before being promoted in spring 2011.
Before this he had a decade-long career as a journalist, including stints as a personal finance reporter for the Daily Mail Group and a freelance financial services commentator for the BBC, Sky and Channel 4.
O'Sullivan said: ‘I really enjoyed my time at Fleishman-Hillard and learnt a huge amount there but felt it time to move on to a more comprehensive value offering. Cubitt has a fantastic team focusing on the financial services area who are very specialist, and it provides great value to clients above and beyond what larger agencies offer.
'I was trained as journalist with the public in mind so I’ve always tried to find a similar driver in PR and it’s always been giving clients good value. A specialist agency allows me to do that.’
Simon Brocklebank-Fowler, managing partner of Cubitt Consulting, said: ‘Alan’s appointment is the sixth senior appointment in as many months, giving us the preeminent bench now in high-end, bespoke corporate comms services in the wholesale financial services space, with emerging capability in retail.’
The appointment follows Cubitt's appointment of College Hill’s Gareth David as a director last week.
David had spent 19 years at College Hill and before that two years as head of external affairs at GKN. Prior to his comms career, David was a business and financial journalist working at the The Times, The Observer and The Sunday Times.
The agency also recently brought in Paul Arvanitopoulos from Mattison Public Relations.
O’ Sullivan's appointment comes after PRWeek revealed that James Murray had resigned from Fleishman-Hillard six months after joining as MD for corporate comms, public affairs and financial services.