forum 7: week of 27 Feb - pragmatic encroachment
Assumption 3 is plausible. If I know O is the best option, then I ought to do O. This is because I have the reason that O is the best option, where having such a reason is bearing some sufficient epistemic relation to that reason.
I'm not convinced that this relation is a relation of 'knowing that', however. When I know r and r is a reason for doing O, then I ought to do O. But the relation I bear to r may just as well be 'believing that' or 'judging it to be the case that'.