The Eircom stalwart, who first joined the company as an engineer in 1975, said he intends to remain within communications, but declined to reveal his plans.
'The company has been through a chequered ten years and I feel I have been through almost everything I can go through,' said O'Sullivan.
'I want to go on and do something different, although I am staying in the same line of work,' he added.
O'Sullivan reports directly to Eircom's recently appointed CEO Philip Nolan, with a remit to oversee a team of 25 covering all internal and external communications, brand management and government affairs.
He said a decision is yet to be made on his replacement: 'We're not recruiting right now, it's something Phil Nolan will deal with down the line.'
O'Sullivan led Eircom's comms through a series of major company changes, including its flotation in 1999 and its sale to the Valentia Consortium led by Sir Anthony O'Reilly last year for £1.9bn.
Eircom was delisted from the London Stock Exchange last December, concluding the Valentia takeover.
The firm sold its mobile arm Eircell to Vodafone before selling to the consortium.
Nolan joined in January from his former post as CEO for the Lattice group.