Marijuana is classified under the federal Controlled Substances Act as a Schedule I drug, which means it is unlawful under federal law to grow, distribute, possess or use marijuana for any purpose. Individuals who engage in such conduct could be subject to federal prosecution.
However, the courts thus far have upheld a state's authority to decriminalize marijuana for state law purposes. Oregon did so for medical marijuana in 1998 and for recreational marijuana in 2014. What that means is someone who grows, distributes, possesses or uses marijuana within the limits of those state acts is immune from state prosecution, but might still be subject to federal prosecution if federal authorities desired to do so.
See a great guide to what is currently legal in Oregon.