LCD 16×2 have around 16 pins which needs to be connected to microcontroller in order to make it work. But by using an I2C extender, we only needs to connect 2 pins (SDA and SCL) to the microcontroller.
This oled display uses the I2C protocol to communicate to the microcontroller. So here you need only 2 pins i.e. SDA and SCL from the microcontroller and..
connect the first pin of the PCF8574 to the VSS of the LCD, 2nd pin to the VCC and so on. The input pins of the PCF8574 i.e. SCL and SDA should be connected with the SCL and SDA of the microcontroller.