It can be hardware or software, but I'm afraid it is both.
Which Arduino board do you use and which modules do you have (can you give a link to where you bought them ?).
The OLED displays have the nasty habit to disturb the I2C bus for others. It is therefor important to know exactly what you have.
What does the Serial Monitor show ? Can you copy that and show it ?
I'm not happy with the DFRobot library. The I2C bus is initialized for every read and write, that is very weird. That library is written as if the DFRobot library is the only library that uses the I2C bus.
Please show the full sketch between code-tags.
What does the compiler say about memory usage for Flash and SRAM (dynamic memory) ?