Ran up to Charlotte today to see my kids and made it up and back on 1 tank of gas.
Gas, btw, was 20/30 cents cheaper then here. Anyway got 31.75 miles to the gallon, which was a surprise, considering mix driving lately has been anywhere from 22mpg-25mpg
Think it's time for a new O2 sensor? Or any ideas why the mileage is so erratic

it's not erratic, it's just that you were driving on the interstate for a long time with little throttle play probably.

is it necessary for the CEL to go off if the O2 is bad? Or can it be bad and CEL not go off?

you have a 4 cylinder 318 .. i would expect that kind of mpg

i recently got 370 miles on one tank and filled up as soon as the light came on.

calculated with the gallons in the tank and the milage got 26.6 mpg. not bad for 145k mile motor i think