Hi, !
You can definitely ask your doctor or pharmacist about contrindications and what you can take with Olanzapine. That said, most of the information I found about taking decongestants or antihistamines with Olanzapine was cautioning that Olanzapine can cause an increase in decongestant/antihistimine side effects, like drowsiness, dizziness, etc.
Again, your best bet is talking to a pharmacist or your doctor, who knows your full medical history and can offer safe advice.
Best, Erin, Team Member.