Oh dear(!) I can't find my book(!) / Oh dear, I can't find my book(!) -- These are the most likely; the exclamation "Oh dear" is a single unit. "(!)" means that you can either use an exclamation mark or a full stop, depending on how exclamatory you want it to sound.
Oh! Dear, I can't find my book(!) -- Possible, but less likely, and has a different meaning. Here "Oh!" is an exclamation by itself, and "Dear" is an affectionate form of address.
(b) Oh! Dear! I can't find my book(!) -- Theoretically possible, but not very likely. "Dear" is again an affectionate form of address, but here you're exclaiming it.
(a) is not really possible.
In your second sentence, both your suggestions are OK and mean about the same.