Hello,
Recently my 98 civic ex stock exhaust fell apart from rust. This is my beater car and it is all stock.
I order a new front exhaust pipe, but the new one does not have a O2 sensor bung. This sensor would be the header sensor.
Do I really need to have the sensor or could I just install new pipe without the sensor?

 

You need the primary O2 sensor. Return the header you bought and find an OEM one.

 

Or take what you bought to an exhaust shop and have them weld an O2 sensor bung in. Although I would go RonJ's route before modifying the (probably) cheap crap you bought.