You're operating outside the OPTs minimum power supply requirements. The datasheet specifies a minimum of +2.25V and -2.25V. You are not meeting that. Pin 1 is only 1,7V above pin 8. Overall, I wouldn't have chosen this part to feature in an all 5V design.

I would also like to point you at the Vref pins on the ADC. They will allow you to map the converter range exactly onto the output range of the OPT, even if you have an awkward ground scheme (connect pin 8 of the OPT to pin 11 of the ADC). What you really need to get the OPT working in this case is a +5V to -5V DC/DC converter.

Then there still remains the general conditions like grounding scheme, power supply decoupling etc.

BTW: why not a simpler solution like a passive LDR? That'll work without any requirement for a symmetrical power supply.