Hi.
You can run the following code to find out the default I2C pins:

void setup() {
  Serial.begin(115200);
  Serial.print("SDA: ");
  Serial.println(SDA);
  Serial.print("SCL: ");
  Serial.println(SCL);
  // put your setup code here, to run once:
}
void loop() {
  // put your main code here, to run repeatedly:
}

I hope this helps.
Regards,
Sara

Hi René,
I just worked on the exact same project ~2 months ago.
Using Sara‘s code you can identify the I2C address of the sensor and display.
You can define suitable pins yourself. – But check the data sheet if a pin is really Input and/or Output.
I assume you are using the 1306 OLED display. – Typically this can be wired SPI or I2C.
I am familiar with that one from joy-it. Here you need to solder a solder dot or resistor.
Mine was factory defined SPI, so I think you need 2 more cables to the ESP.
If you‘re still stuck, let us know what information is missing.
 
Good Luck & happy coding!

LoetLuemmel