I take zyprexa and pristiq (desvenlafaxine) for ocd. the medications cause weight gain and severe sleepiness. if i sleep 10.5 hours, could this harm me long term?

Sedation: You don't share your zyprexa (olanzapine) dose, but you do mention "severe sleepiness." yes, weight gain and inactivity can harm you long-term as obesity and diabetes are more likely to develop -- along with other conditions (cardiovascular disease, etc) that can accompany them. Please tell your psychiatrist about these side effects -- doses may be adjusted accordingly.