Oil pressure sending unit location?

I have a 1998 Chevy Malibu. My oil sending unit is leaking oil. How did this happen and what can I do to fix it. My car has 109,000 miles and it won't start. I didnt have any problems until Jiffy Lube forgot to replace my oil cap.

Thank you for your fast and accurate response. I wanted to make sure this mechanic was doing the right thing. The car still won't start. I do have an anti-theft system problem as well. For months I would try to start my car and the theft system light would come on. I had to wait about ten minutes and it would start right up. Now the car doesn't do that anymore. I have to wait hours before it will start up. Can I use a bypass kit or is the problem more severe?

I replaced the sensor. Which was not super fun and it's a whole lot better. My rpm gauge has stopped working and now it's working again. It smoked a little bit when I started it the first time but that was it. It still had a second of delay to start but that is a whole lot better than it was. Thank you so much for your help. I think replacing the starter is next just to be on the safe side.