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 the VCC and GND.
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.