O'Neill has named his first squad since taking over from Nigel Worthington as national boss earlier this year, but is eager to continue expanding the available player pool. Cullen, 26, was born in Ibaraki, Japan, but his father is Northern Irish and O'Neill has made inquiries to gauge his interest in declaring for his side.
Answering questions on a webchat hosted by the News Letter newspaper, O'Neill said: "An approach to his (Cullen's) club has been made for further information. I intend to watch this player personally or have someone watch him in the near future."
He added: "In the short time I have been in the job, I have actively tried to deepen the player pool available for selection by trying to find players who are eligible for NI.
"This will continue throughout my reign as it will allow our younger players time to develop and not be rushed into full international football."
O'Neill also said that former Manchester United goalkeeper Roy Carroll was under consideration for a return to the international fold.
Carroll fell firmly out of favour under Worthington but has expressed a desire to represent his country again under the new regime.
He is currently at Olympiacos, saving a penalty in the Europa League with his first touch for the club on debut. He was not in O'Neill's 24-man party for the February 29 friendly against Norway but could reappear in the future.
"I have had discussions with Roy and he has confirmed that he wished to be considered for the full international squad again," said O'Neill.
"Unfortunately his club commitments with Olympiacos mean that he would be unable to join the squad until late Monday evening. We both agreed given that he has just moved to Olympiacos that he should concentrate on his club commitments in the short term. He will definitely come into consideration for the next squad."
Source: PA
Source: PA