Do I qualify for the severly disabled credit? If I have a permanent mental disability that requires medication for the rest of my life?

The Oregon Severely disabled exemption is not a credit.  It is an exemption which will lower your taxable income.  In order to claim it you must need special equipment or assistance to partake in normal activities.  Medicine would not count as equipment or assistance.

Physical or mental condition” means an impairment of the body which is of such gravity as to prevent a person from engaging in normal activity without the aid of special equipment or assistance. Examples of severe disabilities include cerebral palsy, multiple sclerosis, and brain damage. These disabilities may or may not be permanent. Disabilities due to surgery, hospitalization, disease, or injury or acute infectious diseases do not qualify where a person can be expected to resume a normal life within a generally accepted recovery period.