Well, to begin with, strictly speaking it’s not os but (a), because it’s an if that means whether (not that people don’t these days often use os here by influence of English). And then - for the sentence given here - it’s ydy/yw, not oes. And then it’s not bo fi, because the (underlying) that which we have here (even though not stated) is the relative that meaning which, not the reported speech that - again confusion because of English, which uses the same word for both of course.
So:
Dw i ddim yn siwr (a) ydy unrhywbeth dw i’n ddweud yn gywir
(Local variations may apply)
(P.S. Oh my goodness - I think people round here are going to LOVE the new book! 
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