help plz. s13 o2 sensor question

[booosted_s13]

93 s13 coupe
how bad is it to run without one? i have heard its ok to run without one, is this true? if not, why. im learning so give me some slack haha

[jdm213]

well o2 sensors do make a difference in gas mileage so get one but its not as neccesary on modded cars that are pushing over their limits cuz they are tuns and run standalone fuel management systems