Replacing an O2-sensor is not a big thing. Neither the part nor work. I'd say it's defenitely worth doing. It may get worse in that sense, that you can not hide the fault when selling the car, and then it reduces the price you get.
Besides I am not sure which O2-sensor you actually use, but if it is the one before the cat, then it may lead to damage, which requires replacing the cat. And then it is much more expensive. In my terminology that sensor would be upper and it is primary because it gives the input to fuel injection. The one after the cat is secondary and exists only to verify the system works. Older cars had only the one before the cat.