I have 2000 expedition and the oil gauge onley works 1/2 the time but I know that the truck has oil pressure could this be bad gauge or the sending unit

 

Hopefully just a bad sender..The sender is nothing more that an 'on/off' switch, meaning withoil pressure the senders switch 'closes' registering a normal pressure on the gauge.. You can try gounding the sender wire WH/RD and the gauge should read a normal pressure....

 

2000 Expedition: Had the same problem with the oil pressure sender, usually when cold. I replaced the sender to fix the problem. The sender is near the oil filter, next to the power steering pump on the 4.6 Liter engine. Easily assessable from under the vehicle. Mileage before failure: 110,000.