The G7 ministers have pledged to locate terrorist funds
Mr O'Neill was asked in a televised interview whether claims could be verified that the amount of funds channelled by suspected terrorists was greater than initially thought.
"We're probably not at the end of our discovery process," Mr O'Neill said.
Mr O'Neill said US authorities were currently in the process of building "spider webs" of financial connections between the suspects involved in the 11 September attacks, their banks and others. Such connections could provide vital evidence in implicating Osama bin Laden in the attacks.
"We're seeing multiple connections and we're finding patterns of behaviours that we didn't know existed before that will give us a basis for refining our search," Mr O'Neill said.
Mr O'Neill also said that international cooperation was proving vital in the search.
"It's going very, very well," Mr O'Neill said.
Global pledge
Mr O'Neill's statements come shortly after finance ministers and central bank governors of the G7 group of industrialised countries have vowed to track down terrorist finances.
They also called for better exchange of information and the supervision of offshore assets to ensure there was no hiding place for terrorist money.
Worldwide laws
The finance ministers plan to use an anti-money laundering agency - the Financial Action Task Force - to help track down the groups financing terrorist groups.
They also called on the International Monetary Fund to supervise offshore financial centres.
Earlier at the meeting, UK Chancellor Gordon Brown called for worldwide laws to close the net on terrorist funding.
He told his colleagues that any rules drawn up must also apply to off-shore tax havens so there was no "weak link" in the chain to cut off the funds.
Mr Brown said all nations should ensure there was no safe hiding place for terrorists or their cash.
He told BBC Radio: "What the discussion at the G7 is all about is how we can agree a plan of action where each of the major countries takes similar actions in relation to the freezing of money and in the exchanging of information where there are suspicions."