i think it is difficult, because with terraDome i can not give you a scenefile. It is not like something that can be reproduced with some primitives.
I try to explain by using an included promoscene callen Mountain View.
In the picture you can see viewport and both renders octane and iray in seperate windows.
The octane picture looks almost like the viewport. So i would say OCDS is not doing something wrong.
But in Iray it looks so totally different.
What i found out meanwhile is that the scaling of the textures are wrong. When i change it in OCDS to x=1 and Y=1 then almost fit more. but i need to try out more there also.
Perhaps you have an idea to help.
I think also the Translation and Rotation does not fit, but it is late here today so i can test around more tomorrow.
My bloody non technical understanding is, that , because in the scene you can change or morhp around the Terrain zones that TD3 will add these transformation information directly before Iray wants them.
Because it also looks this broken in DAZ Viewport.
And as OCDS is importing these data while adding the objects to the scene and not later before rendering again. Something like that. Could be something special of Iray.
But i thought until now that Iray and Octane are technically not so far away from each other - why should NVidia build two totally different Renderengine. I think at least they are based on the same architectural software basis.