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Confusingly though, the former Barnsley player insists he never signed a contract with the Icelandic club.
And club Chairman Steve Thompson told the Nottingham Evening Post that the club has cancelled the Irishman's registration until the matter is resolved.
"There is some confusion over whether he is still contracted to Keflavic in Iceland," said Thompson. "We have asked them to provide proof to back their claims that he is contracted to them until October, which he says he is not. But we have cancelled his registration with us for now.
"If he is contracted with them, we would not be able to sign him until the January transfer window, which is far from ideal.
"Hopefully, it will be sorted out, but as things stand, it is not looking likely."
O'Callaghan made an immediate impression after 'signing' a 12 month contract for the Magpies, scoring on his debut during last week's friendly at Hucknall Town.