However, the Blues supremo revealed the club are prepared if the striker snubs their contract offer, with a list of alternative targets already drawn up.
It’s no secret Danny Cowley is in the market for new frontmen this summer, with no senior options to select from at present.
In the search for bodies, he hasn’t given up hope of bringing Leicester loanee George Hirst and ex-Sunderland man O’Brien back to the club – with the latter being offered fresh terms to extend his stay.
However, with the former Millwall favourite yet to put pen to paper, there’s a growing uncertainty he’ll do so.
The 28-year-old became a firm fans’ favourite after arriving from the Stadium of Light in January – with his form for the Blues seeing supporters clamouring for him to stay.
But with Cowley explaining how the offer currently with O’Brien can’t stay on the table indefinitely, the Blues CEO expressed how it’s imperative they plan for all possibilities regarding his future.
Cullen told The News: ‘O’Brien’s future is a situation that’s still ongoing.
‘I don’t want to comment on any particular players who we are in discussions or negotiations with at the moment.
‘I think it’s best if we do so in the best way we possibly can. It’s very difficult in the goldfish bowl of football to do that and keep those discussions really confidential and private.
‘When we’ve got something to announce we can then go out with it. If it doesn’t it’s important to have those different options lined up which we have in different positions.’
After Pompey completed the midfield signing of Marlon Pack on Wednesday, attention has swiftly turned to bolstering the Blues’ striking options.
Although the need for signings in that department is growing, Cullen stressed the importance of approaching recruitment sensibly and intelligently.
He added: ‘We’ve got to make sure we work in a sensible way and Danny talks about doing so in an intelligent way, which is entirely right.
‘We’ve got to make the best of what we’ve got to get a really good team together that’s capable of challenging in what’s going to be a very tough League One as always next year.’