Three common side effects of
omeprazole are nausea, mild diarrhea, and headache.
Omeprazole also lists as possible
side effects, though not common, of constipation, depression and anxiety.
I remember getting a nasty headache while on omeprazole myself, unlike my usual migraines. The doctor suggested I go off of the drug, see if the headache stopped and go back on it to see if it started up again.