Odometer problem?

Hello, when you say Oil alert light, do you mean the "Change Oil Soon" light? If so that is not the correct procedure for doing so.
Also, there is a Technical Service Bulletin on the Odometers are not accurate or not being able to program the original mileage into a new Instrument Cluster on these vehicles, I'm not sure what the mechanic did, but he may have used a scan tool and gone into the Cluster and possibly erased the memory. Sometimes the memory for the mileage is stored in an EProm in a different module, such as the body control module. In any case, the mileage has to be correct on the vehicle, it is the law, so there a 2 options. I will post some information about this and what you can do, but it sounds like the memory got erased, or are you sure he didn'tt set the odometer to the Trip meter? Where it will count the mileage and can be reset back to zero. Have you tried changing to the Trip odometer and if so, what does that say?

Below, diagrams 1 and 2 are for when the Cluster is replaced, and the correct mileage cannot be program for whatever reason. If it has actually been erased or the data corrupted, you will need to call a dealership and ask about getting an "Odometer Service Label" and the original mileage will be written on it, and it must be put in the driver's door frame.
Diagrams 3 and 4 is a TSB about the Odometer is not accurate sometimes in these vehicles, just for your personal info.
Diagrams 5 and 6 are for resetting the Oil service light. And diagram 7 shows that the mileage for the vehicle may not be stored in the Cluster, its most likely stored in the body control module or PCM (engine computer).

So, if it's not just the trip mileage that it got changed to, see by pushing the button by the mileage if it's on Trip mode. Then you'll need to either get an Odometer service sticker with the original mileage written on it, or have the cluster sent out and have them attempt to reprogram it with the correct mileage.
I don't see how he possibly erased the mileage unless the EEprom was messed with. It is the chip that the mileage is stored on.