What does OE do on TXB0102 device
When the output-enable (OE) input is low, all outputs are placed in
  the high-impedance state.


This means that if you make OE low (connect it to GND) then all outputs are switched off. The chip will do nothing in this state. Basically it is like the chip has been removed from the circuit (I know that's not 100% accurate but it is a simple analogy).

If you make OE high (connect it to VCCA, so 1.8 V in your design) then the chip's output are enabled and everything is working.