So you wind up with the anomaly that our medical marijuana user now has an Oregon CHL and so may carry a handgun concealed without violating Oregon law. BUT, he is a prohibited person under federal law, and therefore commits a federal felony by merely possessing a handgun.
18 USC 922 (g)(3) says it is unlawful for any person who is an unlawful user of or addicted to any controlled substance (as defined in section 102 of the Controlled Substances Act (21 U.S.C. 802)) to possess a firearm.
THC is a schedule I drug. Schedule I drugs may not be prescribed under federal law. Therefore a prescription for marijuana does not render someone a lawful user of marijuana.