If I am interpreting the gov.uk site correctly, I believe we would be allowed to have up to 3000 in the ground.
'and in outdoor venues with a capacity of 4,000 people or half-full (whichever is a lower number). In the largest outdoor seated venues, where crowds can be spread out, up to 10,000 people will be able to attend (or a quarter-full, whichever is lower).
Given that the max at the Kass is 12,000 then we should, I believe, be able to have 3000 attending. Just got to be spread out. No away fans, I'm assuming but that may change. Would make it easier to get 3000 OUFC supporters into the ground.