To be certain, you should:
1. Consult a repair manual: A Haynes or Chilton manual specific to your 1999 Dodge Ram Van with the 318 engine will have a diagram showing the exact location.
2. Use a parts diagram: Many online auto parts stores (like AutoZone, Advance Auto Parts, etc.) allow you to look up parts diagrams for your specific vehicle. Find the oil pressure sensor in the diagram; this will pinpoint its location on the engine.
Looking at the engine itself is the best method once you have a general idea of the location from a diagram. It's usually a relatively small sensor with one or two wires attached. Be cautious when working around hot engine components.