Where is the oil pressure sending unit on a 1999 Honda Prelude H22?

The oil pressure sending unit on a 1999 Honda Prelude with the H22A engine is located on the passenger side (right side) of the engine block, near the oil filter. It's a relatively small sensor, often with a single wire connector. You'll likely need to look closely and maybe even use a flashlight to spot it. It's usually tucked in somewhat, so be prepared to work in a relatively tight space.