The Departmental Museum of the Oise is housed in the former Bishop's Palace, a handsome 16th-century building immediately north of the cathedral in the Rue du Musée. The palace itself is worth seeing: a fine Renaissance courtyard and a medieval undercroft survive intact beneath the later building.
The collection covers regional history, archaeology, and decorative arts from Roman Gaul through to the 19th century. Highlights include medieval sculpture salvaged from the cathedral and nearby churches, Beauvais ceramics and faience from the 18th and 19th centuries, and an archaeology section covering the Gallo-Roman settlement at Caesaromagus (the Roman name for Beauvais). There are also paintings and furniture from the ancien régime period. The museum is well-organised and uncrowded — a good complement to the cathedral if you want to understand what Beauvais was before the bombing.
Check current opening hours before visiting; the museum has reduced hours in winter and is closed on Tuesdays.