O2 Sensor/EGR Solenoid/??

My '98 6 cylindar has its Check Engine light on. Last week this happened and the car started hesitating so I had the EGR Solenoid replaced per my mechanics advise. That worked for a week. Lights back on again. Now he's saying it could be the O2 sensors or something else which he has said yet.

My question, do I need to worry if it is my O2 sensors (he wants $260 to replace) or can I can continue to drive until I find a more reasonable price?

Thanks...Debbie