Depends on how you define a "side effect." Antipsychotics are sedative drugs... saying that sedation is a "side effect" of olanzapine is like saying that platelet inhibition is a "side effect" of aspirin... it's true that aspirin causes platelet inhibition, but it's hard to call it a "side effect" when that's actually the reason why it's often being used.
So, depending on how the question is asked... sedation is more common than weight gain, but weight gain is usually the most common unintended effect. I'd say that weight gain is the most common "side effect", but that's mostly a question of semantics... if you get a question about it on Step 2, it'll probably ask something something more specific. For instance, if it asks "which of the following effects is most likely to be noted?" and sedation is an answer choice, then that's probably right. But if it asks "which of the following long-term effects is most likely?", sedation probably won't be an answer choice and the right answer will be weight gain.