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I'd back up what Greg says here although this really is a minefield for the person learning English, isn't it?

Sentences such as "I thought you ought to know" and "I think you ought to know" don't usually indicate that someone will end up not knowing something.

Likewise, sentences such as "you ought to know better than to do such a thing" or "you ought to be ashamed of yourself" aren't necessarily spoken to such ignorant or shameless people!

"He ought to win with that song, it's the best entry in the contest" could also represent a strong belief in a positive outcome.  "House prices ought to go up by a third" has only a limited sense that something else might happen.

Like the use of "να" in Greek, the implications of "ought to" often have to be discerned and learned within specific contexts.