When the oracle does that weird green speech balloon thing. Anything you'll remember while passing through the memory charm (not counting being banished past it).
Off hand comments the oracle makes when not doing that still might be true, he is an oracle after all, but he'd be under no obligation to tell the truth. Though because he seems to assume his little charm is completely foolproof he probably isn't lying in any of his offhand comments, there would be little to no point.
Spoiler: Crazy Theory
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That is, unless he did in fact know Roy would get past the memory charm when he was banished and so he tried to plant seeds of a specific interpretation of the prophecy about Belkar, for... some reason. Maybe fate isn't fixed and he wants to make sure Belkar never gets raised, or prophecies are sort of a toss up between the interpretations until one of them happens or something. All highly unlikely I think.