I haven't tried the IO Pi on the Orange Pi PC Plus, only the Orange Pi, Orange Pi One and Orange Pi Plus but there shouldn't be any reason why it is not compatible as the GPIO header is the same on the PC Plus model.
Can you try running the command "sudo i2cdetect -y 0" and see if the IO Pi appears on the bus? It should show up on 0x20 and 0x21. If it is not there then there is probably a problem with the I2C configuration and it may be worth posting a question on the orange pi forum to see if there are any additional steps needed on the PC Plus model.
If it does show up then it could be an issue with our IO Pi libraries not looking on the correct I2C port. If you are using our IO Pi python library then you may need to make a small modification to the IOPi.py file as by default it looks for the IO Pi on the I2C port 1 which is what the Raspberry Pi uses where as the Orange Pi models use I2C 0.
On line 137 in the __get_smbus() function you will need to change "i2c__bus = 1" to be "i2c__bus = 0". That should make it look for the IO Pi on the correct I2C bus and hopefully solve your problem.