OK, I admit it. Part 1 was an impressionistic mish-mash of ideas: so let's try to add some precision here.
If the aliens are inscrutable, then I think that means that we fail to come up with what is technically called an intentional description of them. This means we can't seem to model the alien in terms of coherent beliefs, goals and plans.
Note that this doesn't mean that there are no such consistent intentional descriptions. Arguably without such the aliens wouldn't count as agents - intelligent entities - at all. The aliens may in fact be perfectly aware of their own goals, beliefs and plans. The fact that we can't conceptualise the aliens in this way is what makes them incomprehensible to us.
In fact we may recognise the aliens as intentional without any understanding of what their goals, plans and beliefs are either in general or in particular cases. If we don't recognise the aliens as intentional at all, our only recourse will be to classify them as some unpredictable and exotic 'natural phenomenon'.
Now let's move to the universality of OCEAN. I suspect that the issue of alien personality is rather beside the point as regards their comprehensibility, for the reasons already discussed above .. but let's persevere.
Openness dimension: this measures interest in ideas and a more general neophilia. It correlates with IQ. Any entity which can handle symbolic thought and doesn't live in the eternal present, like an insect, is surely somewhere on this dimension.
Conscientiousness: all entities face difficulties along with the option of perseverance or cutting their losses and doing something else. Complex projects extend in time as well as space and do not reward a heavy discounting of the future. If our aliens are worth worrying about, they'll be on the 'C' end of this scale.
Extraversion-Introversion: biology knows this as 'approach vs avoidant' behaviour. Enough said.
Agreeableness: the biological default between two animals is (i) indifference if their niches don't overlap; (ii) fight or flee if they do. Warmth and affection, affiliative behaviour, is a mammalian kin-nurturing trait which has been leveraged in social animals such as humans. If the aliens work together they'll be on this scale. (And if they don't they'll be a lot less scary).
Neurotic-Stable: this captures the superego-ego-id trichotomy which Freud discussed. People who score stable successfully inhibit, in socially-acceptable ways, the moment-by-moment impulses of the unconscious; neurotic people less so. Only if the aliens have our sort of layered brain will this dimension apply.
So it seems that the universality of the five-factor model is reasonably well-founded. But it's slightly beside the point when considering the sheer otherness of incomprehensible aliens.