Why did my ford focus 2012 oil change message come on?

The oil was just changed at the dealership service 2000 miles ago a couple months ago.

It came on because nobody reset it when the oil was changed. Try reading the manual.

Double check your manual for details. I think that is a reminder light that comes on base on miles driven since last reset, ie it is not a sensor. Make sure it is not the low oil warning light.
Dealer most likely forgot to reset it. Manual or online you can find out how to reset it yourself.

Warning message Low Oil

Low oil means you need to check and TOP UP the oil. It is not a service needed. Ignore this and you can seriously damage your engine.

Unless you do an extreme amount of idling (like all day) or had the car in for service that involved reprogramming the instrument cluster, probably wasn't reset as it comes on due to driving habits only or one year 10k miles usually… Av is right on reset procedure however