I think the only requirement is probably length, it's just the system is measuring it inconsistently for some reason.
Seems to be length and number of reviews, but this is unconfirmed too.
It would help if it was shown in the Foundry why a mission has not reached OR-compliance yet- what condition isnt fulfilled - so one would be able to do something against it. As it is it will kill the foundry earlier or later because it makes it too hard for new missions to become qualified.
And - strange idea - there is a OR reward for "compliant" missions, but ... what is the reward for putting time into the evaluation of new missions?
At least there must be an official statement that defines the rules for OR compliance clearly and transparent.