2002 Neon, advice on codes please

Stepdaughter's 2002 Neon w auto tranny had the check engine light on. She took it to Autozone and got the transmission out of range code and the oxygen sensor code. Car is driving fine and has no shifting problems.

Might it be the TCC solenoid or its wires? Or is it the O2 sensor that is causing both codes? Or should she check the tranny fluid to see if it is full and clean?

What should I tell her to do first? Thanks.
Author: tdzik