The 24-year-old this week moved to reassure O'Neill that he was happy on Wearside after agent Guy Vandermissen hinted he may have to leave for a higher-profile club if he is to achieve his ambition of replacing Thibaut Courtois as Belgium's first-choice goalkeeper.
O'Neill said: "We would really like to try to tie him down to a longer contract. There have been talks about that for some time, but I think we would really want to try to firm that up as soon as we could. I think he's receptive to it."
O'Neill however was relaxed about the whole agent situation.
He said: "I hadn't been made aware of it at all and he came up to me to tell me that he thought there was something lost in translation.
"The agent who was working in the office had made some comment about it and really, if you think about what he said - I haven't seen it first hand and therefore, it's unfair of me to make a comment - but if the gist of it was that Atletico Madrid are a better side than Sunderland, well I am not so sure that too many people would disagree with that.
"I haven't a major problem with that. Our job is to try to make us at some time or another as good as them."