Arduino Uno 2 I2C devices problem
I imagine you are using the Adafruit Library, since thats the URL of the product you posted.

If so, in RTClib.h:

#define DS1307_ADDRESS  0x68
#define DS3231_ADDRESS  0x68


And in the OBDs product page:

OBD I2C Address: 0x68

MEMS I2C Address: 0x62


Which obviously causes a conflict.

It seems that they have a fixed I2C address (some devices are configurable via software, others with external pins). So the solution would be to use other RTC, other OBD (or change its address) or use a bus splitter, or use a MCU with more than one I2C bus.