So I am 19 and my girlfriend is 15. I live in Oregon and she lives in Washington. When can she legally consent to having sex? is it 16 or 18? and me living in Oregon does that play a part in anything?

No matter your place of residence, when you are in Oregon, you are held accountable to Oregon laws. When you are in Washington, you are held accountable to Washington law. In Oregon or Washington, a 19 year old can be at risk for criminal prosecution if sexually involved with a 15 year old minor.

In Oregon the age of consent to have sexual contact is 18.
In Washington the age of consent to have sexual contact is 16 or 18. It is 18 if the acting adult is a foster parent, school teacher or school administration employee with authority over the student; or, the adult is 60 months or more older than the 16 or 17 year old.