service engine light on - indicator is o2 sensor. big deal???

[marcx5]

i went to autozone and borrowed their computer thing to find out why my service engine light was on. the machine indicated it was an o2 sensor.

is this a big deal or expensive to fix?? does it NEED to be fixed?

i'm thinking of getting the 2007 x5... so if i can wait and not put more money into my old car, i'd prefer that.

thoughts?

thanks.