Oregon City is about 20 km south of downtown Portland and is easily reached by public transport. The TriMet light rail (MAX) does not reach Oregon City directly, but bus routes — including the 33 and 35 — connect from Clackamas Town Center MAX station. The drive from Portland via Interstate 205 takes around 25 minutes in normal traffic.
From Portland International Airport, the most straightforward option is to drive south on I-205, which takes about 35 minutes. There is no direct train service; Amtrak's Coast Starlight and Cascades stop at Portland Union Station, not Oregon City.
If you are arriving from Seattle or Eugene by Amtrak, get off at Portland and take the bus south. Oregon City is also bikeable from Portland via the Springwater Corridor trail, though it is a substantial ride of around 25 km each way.