Where is the oil pressure sending unit located on a 1999 Buick park avenue ultra?

The oil pressure sending unit on a 1999 Buick Park Avenue Ultra is located on the engine block, near the oil filter. It's a relatively small unit with a single wire connector. You'll likely need to consult a repair manual or online diagrams specific to the 3.8L Series II engine (the most common engine in that model year) for the precise location. The diagrams will show you the exact placement relative to other engine components. The location is generally low enough to require getting under the car for access.