The University and Eugene police departments have gone through periods of both relaxed policies and rigid ones, but they seem to have finally found a reasonable medium. Dorm infractions are punished very gently, but some police officers will readily issues citations for MIPs (minors in possession of alcohol) and break up parties. For big parties, police will usually let the crowd leave and just write up the owner or facilitator. Smaller parties aren't always so lucky, and it's not uncommon for underage drinkers to get busted at these.
On the drug side of the issue, authorities are often much calmer with marijuana because they understand its regularity (and it doesn't tend to contribute to large gatherings and riots). Concerning harder substances, however, punishment depends on the individual officer and the atmosphere of the situation. Don't be surprised to hear about strict enforcement, as authorities have stated that they won't tolerate drug abuse on or around campus—but don't be surprised if you can get away with it, either.