If you're given information with no indication that it's classified, how are you supposed to know? It's possible you saw it earlier and knew it was classified, but otherwise, it's hard to see how you would know.
Nonetheless, a reasonably prudent man in these circumstances ought at least to have looked through the documents before giving them to a third party.
In my experience, if a document is classified, it says so right on it. And, typically, it has a cover sheet that indicates the level of classification. And, if it is more than merely "confidential", you have to specifically sign for the document in a document log. If none of these procedures were followed, i.e. there were no classification markings on documents, no cover sheets and no log of classified documents for which Mr. O'Neill signed, it's hard to see where he is legally at fault. Though, again, ideally, he'd have checked himself before giving them to a third party whose clearance and "need to know" were uncertain.