The Gun Control Act of 1968 prohibits gun sales to individuals who have been committed to a mental institution or "adjudicated as a mental defective." 

As of 2012, only 38 States actually reported such folks to the National Database so they could be denied a firearm.  I have looked and haven't been able to find Oregon's reporting statistics, even though we are one of the 38.  

This Denial Report system is a close as I can get and it just tells us about those who have been denied a firearm and why and what the OSP did about it.

https://www.oregon.gov/osp/ID/Pages/Firearms-Denial-Information-and-Reports.aspx

I have found several states who have only reported a very miniscule number of people, however.