Horst Janssen was born in Hamburg but grew up in Oldenburg, and this museum gives him the attention his obsessive drawings, etchings, woodcuts, and posters deserve. The collection is strongest if you like work on paper: sharp line, strange faces, botanical forms, self-portraits, and a personality that never quite sits still.
The museum also shows broader graphic art and photography exhibitions. It is a better reason to stop in Oldenburg than any single monument.