So this is my 2nd turbo transmission on seperate vehicles that I cannot get the damned o-ring to stop leaking. Shouldn' this be a basic and simple fix?

 

Not really a problem I've seen. The o-ring just slides into the hole. Is this a new o-ring or a reused old one? I've seen replacement o-rings recently that are undersize - apparently a smaller, metric cross section. These don't seal correctly. I've especially seen this on the o-ring that seals the dipstick tube in trans kits.

If this is the original o-ring, they get hard and flattened and should not be reused.