An oil change won't affect the oxygen sensors -- unless the oil that they
put in is heavily contaminated -- totally unlikely.
As for what is causing the problem, I am afraid that we don't know what
problem you are talking about. There is a long list of oxygen sensor
related trouble codes; some may lead to a bad sensor, others to something
entirely different that affects the sensor output.
I am assuming that the check engine light was on, and someone found an
oxygen sensor related code. Post the code here, and you may get some tips.
Honestly, I don't see much purpose though, if she is going to take it to the
dealer. For sure the dealer won't need this group's help in diagnosing the