Olanzapine, that you are taking already, is known to cause weight gain and stimulate appetite. However if it is not doing the same, an assessment may be required. Also, you wrote that you have been taking medications for OCD since an year. How are you now? Are your symptoms under control?
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There are some syrups that can help in appetite stimulation. Fluoxetine itself may cause some appetite suppression, so that needs to be investigated as well.