My check engine light is on, code reveals O2 sensor, my lx470 is 2003,

Sounds about right. 15 - 16 year old vehicle, possibly original oxygen sensor. We see things like that all the time in our shop. You stated "trusted mechanic", I'd agree. Have the work done, he'll do the best he can. You are driving a vehicle that will last, almost forever and probably have not done too many repairs on.