I have a quick example of loading ORCA data and plotting it, available here:
http://nbviewer.ipython.org/6219116As far as I'm aware, there are no other interpolation schemes available in Iris which can handle the multidimensional coordinates of the ORCA grid, so anything other than a simple nearest neighbour cross section extraction, unless I'm mistaken, is probably not available to you out of the box.
Hopefully somebody with more knowledge on the subject can add some extra light on the matter.
HTH